Log an error if overview proxy service isn't available.

Shouldn't be happening.

Bug: 134981174
Test: None
Change-Id: Ibfcc6e713e4efaad32c84a81f6a35f31e49fcd3a

packages/SystemUI/src/com/android/systemui/recents/OverviewProxyService.java

@@ -746,6 +746,8 @@ public class OverviewProxyService implements CallbackController<OverviewProxyLis
         try {
             if (mOverviewProxy != null) {
                 mOverviewProxy.onAssistantVisibilityChanged(visibility);
+            } else {
+                Log.e(TAG_OPS, "Failed to get overview proxy for assistant visibility.");
             }
         } catch (RemoteException e) {
             Log.e(TAG_OPS, "Failed to call onAssistantVisibilityChanged()", e);