处理 unregister 可能的问题

src/android/RedLightScan.java

@@ -91,8 +91,10 @@ public class RedLightScan extends CordovaPlugin {
     }
 
     private void unregisterReceiver() {
+        if (receiver != null) {
             webView.getContext().unregisterReceiver(receiver);
         }
+    }
 
     @Override
     public void onResume(boolean multitasking) {