Add a null protection for getContext() when updating preview in theme/grid fragment.

They call getContext() in a callback, so it's possible that getContext() is called after the fragment is finished.
At this moment, the getContext() will return null.

Bug: 157624922
Change-Id: Id18b721ccd32a82d1c1925128829fbcd00a45b88
diff --git a/src/com/android/customization/picker/grid/GridFragment.java b/src/com/android/customization/picker/grid/GridFragment.java
index a8f6eda..e25e57c 100644
--- a/src/com/android/customization/picker/grid/GridFragment.java
+++ b/src/com/android/customization/picker/grid/GridFragment.java
@@ -212,6 +212,9 @@
         if (mGridOptionPreviewer != null) {
             mGridOptionPreviewer.release();
         }
+        if (getContext() == null) {
+            return;
+        }
         mGridOptionPreviewer = new GridOptionPreviewer(
                 getContext(), mGridManager, mGridPreviewContainer);
         mGridOptionPreviewer.setGridOption(mSelectedOption, mGridManager.usesSurfaceView());
diff --git a/src/com/android/customization/picker/theme/ThemeFragment.java b/src/com/android/customization/picker/theme/ThemeFragment.java
index 1c3b4c1..61f1949 100644
--- a/src/com/android/customization/picker/theme/ThemeFragment.java
+++ b/src/com/android/customization/picker/theme/ThemeFragment.java
@@ -177,6 +177,9 @@
     }
 
     private void updateThemePreviewColorPerWallpaper() {
+        if (getContext() == null) {
+            return;
+        }
         if (mCurrentHomeWallpaper != null && mWallpaperImage.getMeasuredWidth() > 0
                 && mWallpaperImage.getMeasuredHeight() > 0) {
             WallpaperColorsLoader.getWallpaperColors(