I suspect you're seeing this Java bug. The stack trace is the same from javax.swing.plaf.basic.BasicHTML.updateRenderer() onward. The bug report claims that it's fixed in Java 14. VASSAL 3.3 uses Java 14 by default on Windows and Macs, and can use Java 14 on Linux, so you shouldn't see this problem anymore once VASL is able to run on VASSAL 3.3.
The trigger for the bug appears to be an <img> element with an empty src attribute.
The trigger for the bug appears to be an <img> element with an empty src attribute.