Update FrameTools.cpp

m_ToolBar was not being set to NULL (or something other than True) after Destroy(). Then, in CFrame::UpdateGui, it would segfault because m_ToolBar would point to a non-null location, and UpdateGui would go into the if(m_ToolBar) condition, and would try to dereference m_ToolBar which is now nonexistent.

Fixes crashing of Dolphin when switching Themes. The refreshing action is not the prettiest - not sure how to solve that, but at least for now it does not segfault. Thought about adding checks for whether the toolbar was indeed destroyed or not.