Exceptions with MPR rotate example

[knopkem]

This is for version v0.14.1 (it was working before).
Running the MPRRotateExample shows lots of exceptions in the console, e.g.:

    at publicAPI.handleMouseMove (vtkInteractorStyleMPRSlice.js:285)
    at Object.publicAPI.handleMouseMove (vtkInteractorStyleMPRRotate.js:35)
    at Object.<anonymous> (index.js:59)
    at _loop (macro.js:825)
    at Object.invoke [as invokeMouseMove] (macro.js:814)
    at Object.publicAPI.<computed> [as mouseMoveEvent] (index.js:718)
    at HTMLDivElement.publicAPI.handleMouseMove (index.js:427)

[laozhu]

The same issue.