androidtest timeout on runTestOnUiThread

I am doing usual test such as :

public void testButton(){
/* some code to get Button instance */
runTestOnUiThread(new Runnable() {
            @Override
            public void run() {
                button.performClick();
            }
        });
    getInstrumentation().waitForIdleSync();
}

But now test is launched and never ends. Why so ? (I ma using Android Studio)