SystemUI: fix NPE on global actions dialog

SystemUI would crash when accidentaly double tapping reboot (at least when advanced reboot is enabled)

test: manual
Change-Id: I0bdb2beda23b77fccedfb70e05f5105f275d56b9
Signed-off-by: Timi <timi.rautamaki@gmail.com>

Author: trautamaki

packages/SystemUI/src/com/android/systemui/globalactions/GlobalActionsDialog.java

@@ -1288,6 +1288,7 @@ public boolean onLongClickItem(int position) {
 
         @Override
         public void onClickItem(int position) {
+            if (mDialog == null) return;
             Action item = mAdapter.getItem(position);
             if (!(item instanceof SilentModeTriStateAction)) {
                 mDialog.dismiss();