pointer being freed was not allocated, without any new usage

I going through a piece of code here. For testing purposes it shows a window (QWidget) when executed.

When I hit close it returns:

my_object(7082,0x7fff7a538000) malloc: *** error for object 0x7fff5199b9b8: pointer being freed was not allocated

Where my_object is an instance of the class instantiated at a QMainWindow.

There is no new usage in all code I'd wrote. And also no delete call. How is that (pointer being freed) possible? I though not using new explicit I'll be outside the dangerous zone.