Null pointer exception when saving mapping on macOS

[delingren]

VisiCut 2.1, macOS Sequoia (repros on Tahoe too). I haven't tried on other platforms.

1. Load any SVG
2. In "Mapping" tab, pick mapping by stroke color (or any other custom mapping, for that matter)
3. Make some changes, any changes.
4. Click on "save" icon, accept the default name.
5. Observe the error dialog (see attached screenshot)

This is on the top of the call stack:

https://github.com/t-oster/VisiCut/blob/511a28e82d6b28e95754cd0441e53f134c5930e3/src/main/java/de/thomas_oster/visicut/misc/FileUtils.java#L167C7-L167C12

Not sure how output is null in the finally block but not the try block.

[delingren]

Further observations:

1. Looks like saving an existing mapping also causes a NPE.
2. One level below the call stack:

   VisiCut/src/main/java/de/thomas_oster/visicut/managers/FilebasedManager.java

   Line 217 in 511a28e

   FileUtils.copyFile(curThumb, thumb, false);

   which is for saving a thumbnail.