Toolbar item icon is not shown in android

The toolbar item icon has been shown correctly on both Android and iOS devices before updating the xamarin.Forms to the version 4.0.0.0

After updating the xamarin.Forms to version 4.0.0.0, the icon is appeared on iOS but not on Android devices.
I tried in on different android devices with different APIs.

The problem is just the icon because the text is shown correctly.

I have created the toolbar item programmatically as you can see in the following:

        ToolbarItem myTollBar = new ToolbarItem
        {
            Text = "myText",
            IconImageSource = "icon_plus.png",
            Order = ToolbarItemOrder.Primary,
        };


        myTollBar.Clicked += MyTollBar_Clicked;

        ToolbarItems.Add(myTollBar);

P.S.
"Xamarin.Forms" version="4.0.0.425677"
"Xamarin.Android.Support.v7.AppCompat" version="28.0.0.1"

Thank you,