IDE won't start: java.lang.ClassNotFoundException: org.fife.ui.rsyntaxtextarea.TokenMap

[per1234]

Using Arduino IDE 1.8.9 Hourly Build (current at this time) or Arduino IDE 1.9.0 Beta Build 97 Windows 10 64 bit

The Arduino IDE fails to start. arduino_debug.exe output:

Loading configuration...
Initializing packages...
Preparing boards...
java.lang.NoClassDefFoundError: org/fife/ui/rsyntaxtextarea/TokenMap
        at processing.app.syntax.PdeKeywords.<init>(PdeKeywords.java:69)
        at processing.app.Base.onBoardOrPortChange(Base.java:1320)
        at processing.app.Base$11.actionPerformed(Base.java:1514)
        at processing.app.Base.rebuildBoardsMenu(Base.java:1490)
        at processing.app.Base.<init>(Base.java:273)
        at processing.app.Base.main(Base.java:151)
Caused by: java.lang.ClassNotFoundException: org.fife.ui.rsyntaxtextarea.TokenMap
        at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
        ... 6 more

The issue does not occur with the ARM Linux Hourly Build of the Arduino IDE.

I suspect the problem is caused by: f15ddeb