Fix permission review crash if there's no permission to review.

Fixes: 138386074
Test: presubmit & manual
Change-Id: I4d431206742de017c4000d465fedb3033a98d457
diff --git a/src/com/android/packageinstaller/permission/ui/handheld/ReviewPermissionsFragment.java b/src/com/android/packageinstaller/permission/ui/handheld/ReviewPermissionsFragment.java
index 85ec2d3..7c2eca4 100644
--- a/src/com/android/packageinstaller/permission/ui/handheld/ReviewPermissionsFragment.java
+++ b/src/com/android/packageinstaller/permission/ui/handheld/ReviewPermissionsFragment.java
@@ -188,7 +188,10 @@
             preferenceGroups.add(mNewPermissionsCategory);
             preferenceGroups.add(mCurrentPermissionsCategory);
         } else {
-            preferenceGroups.add(getPreferenceScreen());
+            PreferenceScreen preferenceScreen = getPreferenceScreen();
+            if (preferenceScreen != null) {
+                preferenceGroups.add(preferenceScreen);
+            }
         }
 
         final int preferenceGroupCount = preferenceGroups.size();