It looks like the appstore module you are using is not ready for Mendix 8. There is no updated version of this module available in the store, so you'll either have to contact the developer of the module or manually fix the java errors displayed yourself.
Alternative is to remove the module and work without it for now.
This is en error in the app store module. Best would be to submit a ticket so that it can be fixed. You can create a ticket in github https://github.com/jaspervanderhoek/MicroflowScheduledEventManager/issues