UI: Participant assignment dialog turns white after selecting participant

<img width="553" height="504" alt="Image" src="https://github.com/user-attachments/assets/84b6f13f-34a4-4348-ab13-ec7ef3bfc898" />

But when selecting the participant, the dialog turns white:

<img width="507" height="532" alt="Image" src="https://github.com/user-attachments/assets/a8ac748b-f115-439c-bffc-6214289b09e7" />