I have had this exact same bug after converting to 4.3. I have found a workaround but it is still really frustrating.
If you go to Config/DefaultEngine.ini and set
GameDefaultMap
and
EditorStartupMap
= to nothing. You should be able to load up your project with no map open. However you will not be able to load up your default map. I had two maps and if I loaded the non-default map then loaded the default map it opened fine. But opening up the default map made it crash again.
Despite this workaround it is really frustrating I’d really like it to be fixed.