Do not call onReceiveValue on null reference

android/src/main/java/com/flutter_webview_plugin/WebviewManager.java

@@ -52,7 +52,8 @@ class WebviewManager {
                         handled = true;
                     }
                 }
-                mUploadMessageArray.onReceiveValue(results);
+                if (mUploadMessageArray != null)
+                    mUploadMessageArray.onReceiveValue(results);
                 mUploadMessageArray = null;
             }else {
                 if (requestCode == FILECHOOSER_RESULTCODE) {