Fix dark mode NPE

Fixes: 130812424
Test: toggle dark mode via tile after rebooting and dimssing power menu
Change-Id: I230c843f3246661286e2b4f56540da58fe8b0a3c
diff --git a/packages/SystemUI/src/com/android/systemui/globalactions/GlobalActionsDialog.java b/packages/SystemUI/src/com/android/systemui/globalactions/GlobalActionsDialog.java
index 411536c..505957a 100644
--- a/packages/SystemUI/src/com/android/systemui/globalactions/GlobalActionsDialog.java
+++ b/packages/SystemUI/src/com/android/systemui/globalactions/GlobalActionsDialog.java
@@ -438,7 +438,7 @@
     @Override
     public void onUiModeChanged() {
         mContext.getTheme().applyStyle(mContext.getThemeResId(), true);
-        if (mDialog.isShowing()) {
+        if (mDialog != null && mDialog.isShowing()) {
             mDialog.refreshDialog();
         }
     }