Are you able to share your extension inside a CodeSandbox?
By the way:
Your question seems to be about older versions of bpmn-js-properties-panel. While we are always going to try to answer your questions to the best of our abilities, we encourage you to consider using the latest version of bpmn-js-properties-panel. It makes creating your own extensions a lot easier!
To get started with extending the properties panel check out this example: bpmn-js-properties-panel Extension Example. There is also a dedicated example for asynchronous data loading in a select.