Update FrameTools.cpp to fix crash when changing Themes

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.
This should be `nullptr`