Create new connection, have 2 windows opened. First is Editor (or you can open Schema Browser or anything other as well) and second one is Schema Browser
Very important, that you open new Schema Browser.
Make "Schema Browser" active (click on it), change Tables -> Users
Select random user, then click "Alter User" button.
Do random change there, for example, on "Tablespaces" tab, choose another tablespace from the drop down menu, and then you may as well change it back to the one it was.
Then you should have the following sql:
ALTER USER <>
DEFAULT TABLESPACE USERS;
After that you just click OK, and your active window is now changed to Editor, not "Schema Browser"
By the way, now if you repeat the same process again with the same "Schema Browser", then this does not reproduce any more. So that is why you should open new "Schema Browser".