MediaProjectionPermissionActivity: Fix NPE.

Avoid dismissing a null dialog.

Bug: 19492919
Change-Id: I6642bb5194d9818ff3db1266c7c78ed752e23ecf
(cherry picked from commit 3d332f4e67)

packages/SystemUI/src/com/android/systemui/media/MediaProjectionPermissionActivity.java

@@ -114,7 +114,9 @@ public class MediaProjectionPermissionActivity extends Activity
     @Override
     protected void onDestroy() {
         super.onDestroy();
-        mDialog.dismiss();
+        if (mDialog != null) {
+            mDialog.dismiss();
+        }
     }
 
     @Override
@@ -128,7 +130,9 @@ public class MediaProjectionPermissionActivity extends Activity
             Log.e(TAG, "Error granting projection permission", e);
             setResult(RESULT_CANCELED);
         } finally {
-            mDialog.dismiss();
+            if (mDialog != null) {
+                mDialog.dismiss();
+            }
             finish();
         }
     }