How to change back to default toolbar navigationicon

Question

after changing the toolbar navigationicon using

toolbar.setNavigationIcon(R.drawable.abc_ic_ab_back_mtrl_am_alpha);

I can't get it work on to work switch back to the default (burger) icon:

int[] attrs = {android.R.attr.navigationIcon};
TypedArray ta = myContext.obtainStyledAttributes(attrs);
Drawable drawableFromTheme = ta.getDrawable(0 /* index */);
toolbar.setNavigationIcon(drawableFromTheme);
ta.recycle();