When you import the project to GMS2 you need to make sure you use a short project name. GMS seem to crash when you import a project with long names. So instead of MultiplayerAddOnToPlatformerEngine.project import to MultiplayerAddOnToPlatformerEngine as GMS2 project name. You rename the project to something like MA.project import to MA in GMS2.

GMS2 also now defined "delete" as a system function. So you need to rename a local variable named "delete" to "delete_"