Sometimes throws Uncaught Error: Error calling method on NPObject on Android

Question

I am having problems with the Webview in Android and it's JavascriptInterfaces.

I am passing a string to the JavascriptInterface. When debugging it, I receive the correct string within my Android application. The problem: Sometimes I get an Uncaught Error: Error calling method on NPObject.

Does anybody know why?

The Interface in Java:

public class JSInterfaceGame extends JSInterface {

@JavascriptInterface
public void setShareText(String share){
    shareText = share;
    if(mJSInterfaceListener != null)
        mJSInterfaceListener.onParametersChanged(SHARE_TEXT);
}

The initialization in the onCreateView-Method within the Fragment:

online = (WebView) rootView.findViewById(R.id.online);
online.setWebViewClient(new WISWebviewClient() {
  @Override
  public void onStatusChanged(final WebView view, int progress, long duration) {
    //unrelated
  }
});

WebSettings ws = online.getSettings();
ws.setJavaScriptEnabled(true);
ws.setUserAgentString(USER_AGENT);
ws.setCacheMode(WebSettings.LOAD_DEFAULT);
ws.setRenderPriority(WebSettings.RenderPriority.HIGH);

SharedPreferences settings = getActivity().getSharedPreferences(GameActivity.PREFERENCES, Context.MODE_PRIVATE);

mJSInterface = new JSInterfaceGame();
mJSInterface.setJSInterfaceListener(this); // Defined elsewhere in this class.
mJSInterface.setPlayerName(settings.getString(GameActivity.PREFS_PlAYERNAME, null));
online.addJavascriptInterface(mJSInterface, "JSInterface");
online.loadUrl("http://myurl.something");

Call in Javascript:

function makeShareText() {
  var text = "Some text";
  console.log(typeof text); // Always a string.
  JSInterface.setShareText(text);
}

Answer 1

It happens when you try, using method called from javascript interface, to interact with UI. To solved it in this way:

class mJSInterface() {  public void myFunction() {     runOnUiThread(new Runnable() {              public void run() {                 //Code that interact with UI             }         });      }  } 

Answer 2

To highlight the comment from @Leog

The same error occurs if you call the native javascript function with wrong parameters

This was the source of my error

Answer 3

Another reason can be a RuntimeException on a WebViewCoreThread. Any exception occurred after receiving @JavascriptInterface call will be logged as NPObject error if still running on a WebView thread. Overall insufficient trace message with little clue about the problem.

Correct your issue with handling javascript interface call on a suitable thread.

Example A. (NPObject error):

@JavascriptInterface
public void jsCall() {
    Log.v(TAG, "Prepared NullPointerException on "+Thread.currentThread());
    String s = null;
    s.length();  // This will cause NPObject error
}

Example B. (NullPointerException):

@JavascriptInterface
public void jsCall() {
    new Thread(new Runnable() {
        @Override
        public void run() {
            Log.v(TAG, "Prepared NullPointerException on " + Thread.currentThread());
            String s = null;
            s.length();  // This will throw NullPointerException
        }
    }).start();
}

Take this as an addition to @Nico.S's answer.