why do I sometimes get a javascript error of "uncaught exception: java.lang.NullPointerException"?

Why there are some times while doing the same actions in my app I get
this javascript error:
uncaught exception: java.lang.NullPointerException

I have no idea where it is coming from since there is no line number
and have no idea what is causing it.

how can I trace it and fix it?
and why does it show up in some times and in other times it doesn't?

thanks.

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