Fix dark mode NPE

Fixes: 130812424
Test: toggle dark mode via tile after rebooting and dimssing power menu
Change-Id: I230c843f3246661286e2b4f56540da58fe8b0a3c

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

@@ -438,7 +438,7 @@ public class GlobalActionsDialog implements DialogInterface.OnDismissListener,
     @Override
     public void onUiModeChanged() {
         mContext.getTheme().applyStyle(mContext.getThemeResId(), true);
-        if (mDialog.isShowing()) {
+        if (mDialog != null && mDialog.isShowing()) {
             mDialog.refreshDialog();
         }
     }