Hard to co-relate the image to the code so do your object references follow this syntax?
Forms![main navigation form].[NavigationSubform].Form
Access defaults the name of the subform to [main navigation form] and the sub container name to [NavigationSubform] IIRC. I'd also question the space that you have rpt as in rpt .filter
Hopefully you're also aware that a nav form only loads one form at a time, so if you switch tabs, one closes and another opens so not possible to refer to another navigation subform from another. Probably not the reason why it complains that the object is closed or doesn't exist - more likely your syntax I'd say.
Not sure I see the value in the first few lines of code when you can just have
Forms![main navigation form].[NavigationSubform].Form.Filter = Me.comboBox_FilterByID
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.