[BUG] Library creation when closing loading Bar is buggy

[Thesacraft]

There are 2 issues i found with creating a library the 2nd one is based on the first one.

1. If the Library is created and the loading window for the files is forced closed(alt+f4) the library is still created.

2. If a Library(with many files) is created and the window is closed while the number of files goes up and another smaller library is created, the bigger one is still loaded and Displayed. If now the library save button is pressed the files of the bigger one is saved in the library json file of the smaller one.

Steps to Reproduce:

1. Create a Library with many files (>100000 otherwise you probably are to slow)
2. Force Close the loading bar window (alt+f4)
3. Create a small library somewhere else
4. Wait for a change of the thumbnail icons after the small library is created (indicates that the creation of the big library is finished)
5. Save library