Closing tabs in debug mode

[hojjatjafary]

Hi, Seems closing tabs causes runtime assertion of an invalid iterator here :

https://github.com/chigraph/chigraph-gui/blob/f66c57369a067a66d99e60ed6ddf82a9540a13b2/src/centraltabview.cpp#L246

The item is erased from std::map so the iterator will be invalid but you are trying to compare the iterator with the end of map, it is better to put the assertion before erasing the items.