Show popup in UI thread

app/src/main/java/org/transdroid/core/gui/navigation/NavigationHelper.java

@@ -92,6 +92,8 @@ public class NavigationHelper {
 			return true;
 		if (!ActivityCompat.shouldShowRequestPermissionRationale(activity, permission)) {
 			// Never asked again: show a dialog with an explanation
+			activity.runOnUiThread(new Runnable() {
+				public void run() {
 					new MaterialDialog.Builder(context).content(R.string.permission_readtorrent).positiveText(android.R.string.ok)
 						.onPositive(new MaterialDialog.SingleButtonCallback() {
 							@Override
@@ -99,6 +101,8 @@ public class NavigationHelper {
 								ActivityCompat.requestPermissions(activity, new String[]{permission}, requestCode);
 							}
 						}).show();
+				}
+			});
 			return false;
 		}
 		// Permission not granted (and we asked for it already before)