Fix a NPE when forget wifi network

To prevent the null activity. If activity is null, there is no need to
execute.

Bug: 184154325
Test: manual test
      make RunSettingsRoboTests ROBOTEST_FILTER=WifiDetailPreferenceController2Test
Change-Id: I0dc32e7ea2a0d5a3f2a55ad8936578ed9fadd201

tests/robotests/src/com/android/settings/wifi/details2/WifiDetailPreferenceController2Test.java

@@ -1266,6 +1266,18 @@ public class WifiDetailPreferenceController2Test {
                 Settings.Global.WIFI_DEVICE_OWNER_CONFIGS_LOCKDOWN, 1);
     }
 
+    @Test
+    public void forgetNetwork_activityGone_ignoreFinish() {
+        setUpForConnectedNetwork();
+        setUpSpyController();
+        displayAndResume();
+        when(mMockFragment.getActivity()).thenReturn(null);
+
+        mForgetClickListener.getValue().onClick(null);
+
+        verify(mMockActivity, never()).finish();
+    }
+
     @Test
     public void forgetNetwork_standardWifiNetwork_forget() {
         setUpForConnectedNetwork();