Fix NPE when calendar permissions are revoked

app/src/main/java/org/tasks/activities/CalendarSelectionActivity.java

@@ -34,15 +34,11 @@ public class CalendarSelectionActivity extends ThemedInjectingAppCompatActivity
 
   private void showDialog() {
     FragmentManager fragmentManager = getSupportFragmentManager();
-    CalendarSelectionDialog fragmentByTag =
-        (CalendarSelectionDialog)
-            fragmentManager.findFragmentByTag(FRAG_TAG_CALENDAR_PREFERENCE_SELECTION);
-    if (fragmentByTag == null) {
+    if (fragmentManager.findFragmentByTag(FRAG_TAG_CALENDAR_PREFERENCE_SELECTION) == null) {
       Intent intent = getIntent();
-      fragmentByTag = newCalendarSelectionDialog(intent.getStringExtra(EXTRA_CALENDAR_NAME));
-      fragmentByTag.show(fragmentManager, FRAG_TAG_CALENDAR_PREFERENCE_SELECTION);
+      newCalendarSelectionDialog(intent.getStringExtra(EXTRA_CALENDAR_NAME))
+          .show(fragmentManager, FRAG_TAG_CALENDAR_PREFERENCE_SELECTION);
     }
-    fragmentByTag.setCalendarSelectionHandler(this);
   }
 
   @Override

app/src/main/java/org/tasks/activities/CalendarSelectionDialog.java

@@ -2,6 +2,7 @@ package org.tasks.activities;
 
 import static com.google.common.collect.Lists.transform;
 
+import android.app.Activity;
 import android.app.Dialog;
 import android.content.DialogInterface;
 import android.os.Bundle;
@@ -82,6 +83,13 @@ public class CalendarSelectionDialog extends InjectingDialogFragment {
         .show();
   }
 
+  @Override
+  public void onAttach(Activity activity) {
+    super.onAttach(activity);
+
+    handler = (CalendarSelectionHandler) activity;
+  }
+
   @Override
   public void onResume() {
     super.onResume();
@@ -103,10 +111,6 @@ public class CalendarSelectionDialog extends InjectingDialogFragment {
     component.inject(this);
   }
 
-  public void setCalendarSelectionHandler(CalendarSelectionHandler handler) {
-    this.handler = handler;
-  }
-
   public interface CalendarSelectionHandler {
 
     void selectedCalendar(AndroidCalendar calendar);