Now IF I use <field name="target">current</field> in model="ir.actions.act_window". The view, the function and the button works just fine. Everything works properly.

But WHEN I use <field name="target">new</field> instead, there is an error which can be triggered with scrolling down the new window. Yes the error triggered by SCROLLING DOWN the scroll bar on the new window and then pressing the button. If I don't scroll the window, the error won't show up.