Be careful not to CloseWindow() a window during a while(msg=GetMsg(...))
loop on that window's port (next GetMsg() would be on freed pointer).
Also, use ModifyIDCMP(NULL) with care, especially if using one port with
multiple windows.  Be sure to ClearMenuStrip() any menus before closing a
window, and do not free items such as dynamically allocated gadgets and
menus while they are attached to a window.  Do not reference an
IntuiMessage's IAddress field as a structure pointer of any kind before
determining it is a structure pointer (this depends on the Class of the
IntuiMessage).  If a crash or problem only occurs when opening a window
after extended use of your program, check to make sure that your program
is properly freeing up signals allocated indirectly by CreatePort(),
OpenWindow() or ModifyIDCMP().