Launcher problemsHMCL Crash: Not on FX Application Thread
What it looks like in the log
java.lang.IllegalStateException: Not on FX application thread; currentThread = ForkJoinPool.commonPool-worker-2
How to fix it
- Close the error window and reopen HMCL; because this is a timing race condition during startup, simply restarting the launcher usually bypasses it.
- Update HMCL to the latest release from the Launcher Settings tab (or download the newest build from the official HMCL website/GitHub) to get fixes for background UI thread bugs.
- If you are running HMCL itself on bleeding-edge Java versions (like Java 25 or 26) and see frequent UI crashes, launch HMCL using a Long-Term Support release such as Java 17 or Java 21 instead.
| Loaders | Vanilla, Forge, Neoforge, Fabric, Quilt |
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| Severity | Breaks something |
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| Category | Launcher problems |
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Check your own log
