When Panel Refreshes Or Invalidates - It Randomly Fills Undo Stack With Threads

I am developing an application, for a university project, which is using GDI to draw a flag, my application is also equiped with an undo stack, which is, essensially, where the problem is, so what happens is: I push the draw button to draw a box, when I do that, the solution normally creates the box and stores the object to the undo stack; however, when I ask it to undo the box, and refresh the panel, it randomly adds, endless threads to the undo stack, additionally, everytime that I set the panel to refresh in the undo procedure, it affects the draw procedure; therefore, when I tell it to draw a box, it fills the undo stach with five threads at a time, sometimes, it fills it with much more than five, and sometimes it keeps filling up the undo stack, untill I stop the debugging proccess. This is my code:

[Code]...