Hi Blazej,
Greetings from Syncfusion support.
We have checked the attached video and the code snippet. In that video, we found that list elements are not loaded when opening the popup. This will occur if the fields are not mapped properly. So we suggest you to ensure that the fields are mapped correctly. Also in the provided code files, there is only ListBox and not Multiselect. So kindly share some more details about this issue.
- Fields and Datasource values for the Multiselect.
- Control rendering part and its associated callbacks.
Please provide the above information that will help us to validate this issue further and provide you a better solution from our end.
Regards,
Jeyanth.