Solved the problem on my own. After reading the doc again, I found out that a Toolbar can only contain only ToolButtons, ToggleToolButtons and RadioToolButtons. To add a Combobox or any other item to the Toolbar it must be added to a ToolItem first. Here is the code that changed:
ToolItem container = new ToolItem();
_tb.add(container);
container.add(cb);
cb.show();