Description of problem: Event Source creation fails if user selects no app group and switch to yaml and then to form

Prerequisites (if any, like setup, operators/versions): Install Serverless operator

Steps to Reproduce

1. Make sure an application group exists in selected namespace
2. go to Add -> EventSource
3. select PingSource
4. for application select no application group go to Yaml View
5. Switch to form and try creating EventSource

Actual results: EventSource creation fails with below error

Expected results: EventSource creation should succeed 

Reproducibility (Always/Intermittent/Only Once):

Build Details:

Additional info: