openlayers - Why ol3 google maps is blurred?

I'm developing an application using GeoExt 3(Openlayers 3.15.0 and Extjs 6.0.2). I want to use Google map as base layer using ol3 google maps.

I add ol3 google maps to my project as follow:
index.html:

Now I add google map as follow:

var googleLayer = new olgm.layer.Google();
var map = new ol.Map({
    layers: [googleLayer],
    view: view
});

For activating google map, I use follow code:

var olGM = new olgm.OLGoogleMaps({map: map}); // map is the ol.Map instance
olGM.activate();

Occur an error as follow:

ol3gm.js:47 Uncaught TypeError: Cannot read property 'removeChild' of null

When I set time out as follow, It don't occur any error:

setTimeout(function(){
    var olGM = new olgm.OLGoogleMaps({map: map});
    olGM.activate();
}, 2000);

But in this state The map is blurred. See the result:

But the ol3 google maps sample result at here is as follow:

Why my result is blurred? Where is the problem?

update:

All things in Firefox is OK, This result is in Google Chrom 50.0.2661.102