I create customized Tasks in VueJs depends of that example bpmn-js-example-custom-elements and i had that issue Cannot read properties of undefined (reading 'isGeneric') when i want to bpmn.saveXML() any solutions for that
Could you explain the steps to reproduce your issue in a structured and detailed manner? Thanks 
For me it looks like miss-use trying to attach extensions to the BPMN 2.0 XML.
I had a problem similar to yours, The error occurs because there is no corresponding definition in moddle, I ended up using a custom moddle file to solve this problem, I don’t know if I can help you：