Check callManager reference is still valid in ListenableFutureTask callbacks.

app/src/main/java/org/thoughtcrime/securesms/service/WebRtcCallService.java

@@ -1456,31 +1456,37 @@ public class WebRtcCallService extends Service implements CallManager.Observer,
 
     @Override
     public void onSuccessContinue(V result) {
+      if (callManager != null) {
         try {
           callManager.messageSent(getCallId());
         } catch (CallException e) {
           callFailure("callManager.messageSent() failed: ", e);
         }
       }
+    }
 
     @Override
     public void onStateChangeContinue() {
+      if (callManager != null) {
         try {
           callManager.messageSent(getCallId());
         } catch (CallException e) {
           callFailure("callManager.messageSent() failed: ", e);
         }
       }
+    }
 
     @Override
     public void onFailureContinue(Throwable error) {
       Log.w(TAG, error);
 
+      if (callManager != null) {
         try {
           callManager.messageSendFailure(getCallId());
         } catch (CallException e) {
           callFailure("callManager.messageSendFailure() failed: ", e);
         }
+      }
 
       if (activePeer == null) {
         return;