Can not undo loaded PDB until made at least one change

[kevre91]

Version and Platform (required):

• Binary Ninja Version: 3.1.3570-dev
• OS: Windows
• OS Version: 10
• CPU Architecture: x64

Bug Description: Binary ninja can not undo loaded PDB.

Steps To Reproduce:

1. Open a file which have a PDB
2. Click Plugins->PDB->Load
3. Try to click Edit->Undo which is disabled
4. Click on any variable or function and press n and then ENTER. You don't need to modify it really
5. Now you can click Edit->Undo
6. It will undo loaded PDB

Expected Behavior: I expect it to unload PDB without making any other changes into database.