Update loading-bar.js

[pellanda-atos-ext]

Sometimes, when you reload a webpage, you get an exception "cannot get property lastChild or undefined", because the variable parent is undefined. If that is the case we return from the function to avoid the exception

What issue is this PR resolving? Alternatively, please describe the bugfix/enhancement this PR aims to provide

Have you provided unit tests that either prove the bugfix or cover the enhancement?

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