Try the following way--

public static List <String> fromSDCard(Activity a, String title, String message)
 {
    List <String> listChurchWall = new ArrayList <String>();
    // listing Wallpaper using church names
    String string = "/mnt/sdcard/Church/Wallpaper/";
    f = new File (string+name+"/");
    if (f.exists()) 
    {
      files = f.listFiles ();
    }
    else
    {
      AlertDialog.Builder dialog = new AlertDialog.Builder(a);
      dialog.setTitle(title);
      dialog.setMessage(message);
      dialog.setNeutralButton("OK", null);
      dialog.create().show();      
    }                          
    return listChurchWall;  
 }   

Then in your class do---

public MyActivity extends Activity
{
    ....

    private Method()
    {
         List<String> list = fromSdCard(this, "Your Title", "Your message");
    }

    ....
}

UPDATE:

You get a NullPointerException because something is null that shouldn't be. It happens while sorting the array, so perhaps one of the array elements is null. Take a look at how you assign values to your array.

Perhaps at the top of it, see if any of the objects Object o1 or Object o2 themselves are null.

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public static List<String> fromSDCard(Context mContext) {
        List<String> listChurchWall = new ArrayList<String>();
        // listing Wallpaper using church names
        String string = "/mnt/sdcard/Church/Wallpaper/";
        f = new File(string + name + "/");
        if (f.exists()) {
            files = f.listFiles();
        } else {
            AlertDialog.Builder builder = new AlertDialog.Builder(mContext);

            int imageResource = android.R.drawable.stat_sys_warning;
            Drawable image = mContext.getResources().getDrawable(imageResource);

            builder.setTitle("title").setMessage("your Message").setIcon(image).setCancelable(false).setNeutralButton("Ok", new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                }
            });

            AlertDialog alert = builder.create();
            alert.setCancelable(false);
            alert.show();
        }
        return listChurchWall;
    }

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This is better use this...

public boolean isOnline() 
{
    ConnectivityManager cm =
        (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo netInfo = cm.getActiveNetworkInfo();
    if (netInfo != null && netInfo.isConnectedOrConnecting())
    {
        return true;
    }


   // Toast.makeText(getBaseContext(), "Internet is not Connected", Toast.LENGTH_LONG).show();
    alertwifi();
    return false;
}





 public void alertwifi()
    {
    AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(
            context);

        // set title
        alertDialogBuilder.setTitle("NO Internet Connection!!");

        // set dialog message
        alertDialogBuilder
            .setMessage("Click below to turn on Wifi or Enable Data pack")
            .setCancelable(false)
            .setPositiveButton("Wifi",new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog,int id) {
                    // if this button is clicked, close
                    // current activity
                    wifi();
                }
              })
               .setNeutralButton("GPRS",new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog,int id) {
                    // if this button is clicked, close
                    // current activity
                    datapack();
                }
              })
            .setNegativeButton("Back",new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog,int id) {
                    // if this button is clicked, just close
                    // the dialog box and do nothing

                    exit();


                }
            });





public void exit()
{
     Intent intent = new Intent(Intent.ACTION_MAIN);
     intent.addCategory(Intent.CATEGORY_HOME);
     intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
     startActivity(intent);
}


// create alert dialog
        AlertDialog alertDialog = alertDialogBuilder.create();

        // show it
        alertDialog.show();
    }

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Try this

@Override
 protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.row_item);
Button bDR = (Button) findViewById(R.id.bDR);
bDR.setOnClickListener(new OnClickListener(){

    @Override
    public void onClick(View v) {
        AlertDialog.Builder builder = new 
         AlertDialog.Builder(DeleteRenameList.this).create();
        builder.setCancelable(true);
        builder.setMessage("Would you like to delete or rename the list?");
        builder.setNegativeButton("Delete", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {


            }
        });
        builder.setNeutralButton("Rename", new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {


            }
        });

    AlertDialog.show();

    }

    });

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Follow the steps

1) Create Utility.java

import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.graphics.drawable.Drawable;


public class Utility {
    public interface AlertNeutral {

        public abstract void NeutralMathod(DialogInterface dialog, int id);

    }

    public static void showAlert(final Context mContext, final String title, final String msg, final String buttonCaption, final boolean isCancelable, final AlertNeutral target) {
        try {

            ((Activity) mContext).runOnUiThread(new Runnable() {

                @Override
                public void run() {
                    AlertDialog.Builder builder = new AlertDialog.Builder(mContext);

                    int imageResource = android.R.drawable.stat_sys_warning;
                    Drawable image = mContext.getResources().getDrawable(imageResource);

                    builder.setTitle(title).setMessage(msg).setIcon(image).setCancelable(false).setNeutralButton(buttonCaption, new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int id) {
                            target.NeutralMathod(dialog, id);
                        }
                    });

                    AlertDialog alert = builder.create();
                    alert.setCancelable(isCancelable);
                    alert.show();
                }
            });
        } catch (Throwable e) {
            e.printStackTrace();
        }
    }
}

2) Call the above function from Any Activity you want.

import android.app.Activity;
import android.content.DialogInterface;
import android.os.Bundle;

import com.example.demo.Utility.AlertNeutral;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Utility.showAlert(this,"Demo Alert Title", "Demo Alert Message", "Demo Button Ok", true, new AlertNeutral() {

            @Override
            public void NeutralMathod(DialogInterface dialog, int id) {

            }
        });
    }


}

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