Steps to reproduce:

1. Update Ibexa DXP v3.3.4
2. Edit or Create Landingpage

 
Result:
Error: Undefined Index: editorMode

To quickly fix the error: 

• edit landingpage Content Type 
• edit fieldtype landingpage attributes
• attribute: editorMode: change - save - change back - save the content type again
• you can edit or create landingpage as expected

Expected result:

Site should be ediitable or created with no error