Fix NPE crash in UsageAccessDetails

mPackageInfo will be null in RefreshUI method in UsageAccessDetails.java

Bug:https://issuetracker.google.com/issues/65872768
Test:As explained in the link above

Change-Id: I8bd4b822cfe5d8a3347ca7f5886605cbdfb9b8b6
Signed-off-by: tiansiming <tiansiming@xiaomi.com>

tests/robotests/src/com/android/settings/applications/UsageAccessDetailsTest.java

@@ -17,6 +17,7 @@
 package com.android.settings.applications;
 
 import android.content.Context;
+import android.os.RemoteException;
 
 import com.android.internal.logging.nano.MetricsProto;
 import com.android.settings.SettingsRobolectricTestRunner;
@@ -65,4 +66,11 @@ public class UsageAccessDetailsTest {
         verify(mFeatureFactory.metricsFeatureProvider).action(any(Context.class),
                 eq(MetricsProto.MetricsEvent.APP_SPECIAL_PERMISSION_USAGE_VIEW_DENY), eq("app"));
     }
+
+    @Test
+    public void refreshUi_nullPackageInfo_shouldNotCrash() throws RemoteException {
+        mFragment.mPackageInfo = null;
+        mFragment.refreshUi();
+        // should not crash
+    }
 }