Nullcheck on stop() (wix#666)

Avoid NPE when DetoxServerUrl and DetoxSessionId are undefined

(cherry picked from commit 9ee1723)

detox/android/detox/src/main/java/com/wix/detox/DetoxManager.java

@@ -86,7 +86,9 @@ public void run() {
                 stopping = true;
                 ReactNativeSupport.currentReactContext = null;
                 ReactNativeSupport.removeEspressoIdlingResources(reactNativeHostHolder);
-                wsClient.close();
+                if (wsClient != null) {
+                    wsClient.close();
+                }
                 Looper.myLooper().quit();
             }
         });