Open Source Document Management System | OpenKM

Hi all,
I need help on the code, how to display an error popup on the screen.
I am editing the class openkm/module/db/DbDocumentModule.java
More in detail the method public InputStream GetContent(String token, String docId, boolean checkout, boolean extendedSecurity)

I enclose an example of the popup I would need

Please find attached an example.
https://ibb.co/VCvyGvY

Anyone can help me?

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