Cannot invoke "com.thoughtworks.qdox.model.JavaField.getName()" because "field" is null

[Reissner]

I write a plugin (https://github.com/Reissner/latex-maven-plugin)
and mvn clean deploy results in following message:

[INFO] — plugin:3.9.0:descriptor (default-descriptor) @ latex-maven-plugin —
[INFO] Using 'UTF-8' encoding to read mojo source files.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 14.056 s
[INFO] Finished at: 2023-06-22T09:29:29+02:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-plugin-plugin:3.9.0:descriptor (default-descriptor) on project latex-maven-plugin: Execution default-descriptor of goal org.apache.maven.plugins:maven-plugin-plugin:3.9.0:descriptor failed: Cannot invoke "com.thoughtworks.qdox.model.JavaField.getName()" because "field" is null -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException

After long search (1 night) I found the following 'reason':

package eu.simuline.m2latex.core;
public enum Target {
txt() {
private final String TXT_OUTPUT_FILES = "^T$T
.txt$";

// may throw BuildFailureException TEX01
public void processSource(LatexProcessor latexProcessor,
LatexMainDesc desc) throws BuildFailureException

{ latexProcessor.processLatex2txt(desc); }
public String getPatternOutputFiles(Settings settings)

{ return "^T$T\\.txt$"; }
}, ...;
...
}

Removing the line

private final String TXT_OUTPUT_FILES = "^T$T.txt$";

'resolves' the problem.
I feel this could indicate not just a bug but a conceptual flaw.