bpmn-js@8.10 is using diagram-js@7.8 under the hood, and the Canvas.findRoot was added via diagram-js@8. Maybe you have an implementation mismatch here.
Are you able to share your setup inside a CodeSandbox? This way it would be easier for us to check what’s going on.
many thanks for the very fast replies from your side.
I discovered a very strange situation. In my custom bundle the code area ‘Selection.prototype.select’ was integrated twice and one times with ‘canvas.findRoot()’. After I run ‘npm update’ I got a lot of changes and now no further place with ‘canvas.findRoot()’. I guest this issue is solved.