[BUG] Add virtual machine dialog closes on error

[daviunic]

Describe the bug
The Add a virtual machine dialog will display an error if you try to create a new VM with a name that's already used by an existing VM (this is a design limitation and expected result). However, when you click OK on the error messagebox, the parent dialog closes instead of remaining open like it should.

Confirmed with 86Box Manager version 1.7.4.

[daviunic]

Same problem also with the error messagebox when the import path is empty.