Concurrency problem?

[egonw]

Stacktrace during compiling/testing:

Detected concurrent modification
Modification was done by
java.lang.Throwable
        at org.pathvisio.libgpml.debug.DebugList.add(DebugList.java:145)
        at org.pathvisio.libgpml.debug.TestDebug.test1(TestDebug.java:56)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at junit.framework.TestCase.runTest(TestCase.java:177)
        at junit.framework.TestCase.runBare(TestCase.java:142)
        at junit.framework.TestResult$1.protect(TestResult.java:122)
        at junit.framework.TestResult.runProtected(TestResult.java:142)
        at junit.framework.TestResult.run(TestResult.java:125)
        at junit.framework.TestCase.run(TestCase.java:130)
        at junit.framework.TestSuite.runTest(TestSuite.java:241)
        at junit.framework.TestSuite.run(TestSuite.java:236)
        at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:90)
        at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:365)
        at org.apache.maven.surefire.junit4.JUnit4Provider.executeWithRerun(JUnit4Provider.java:273)
        at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:238)
        at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:159)
        at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:384)
        at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:345)
        at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:126)
        at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:418)