Fix the null pointer exception after applying a wallpaper.

Bug: 183385553
Test: Manual
Change-Id: I769dc8914281a1d393038a1afa9cb0cc989e7071
diff --git a/src/com/android/customization/picker/CustomizationPickerActivity.java b/src/com/android/customization/picker/CustomizationPickerActivity.java
index 5ca00c5..e198484 100644
--- a/src/com/android/customization/picker/CustomizationPickerActivity.java
+++ b/src/com/android/customization/picker/CustomizationPickerActivity.java
@@ -437,6 +437,9 @@
     @Nullable
     @Override
     public CategorySelectorFragment getCategorySelectorFragment() {
+        if (mWallpaperCategoryFragment == null) {
+            return null;
+        }
         return mWallpaperCategoryFragment.getCategorySelectorFragment();
     }