IllegalArgumentException WorkingSetModel.setWorkingSets

1452 tests failed in
https://download.eclipse.org/eclipse/downloads/drops4/I20241217-1800/testresults/html/org.eclipse.jdt.ui.tests_ep435I-unit-win32-x86_64-java21_win32.win32.x86_64_21.html

most like this:
```
org.opentest4j.MultipleFailuresError: Multiple Failures (2 failures)
org.eclipse.core.internal.resources.ResourceException: Resource '/TestSetupProject' does not exist.
org.eclipse.jdt.core.JavaModelException: TestSetupProject does not exist
Suppressed: org.eclipse.core.internal.resources.ResourceException: Resource '/TestSetupProject' does not exist.
at org.eclipse.core.internal.resources.Resource.checkExists(Resource.java:364)
at org.eclipse.core.internal.resources.Resource.checkAccessible(Resource.java:238)
at org.eclipse.core.internal.resources.Project.checkAccessible(Project.java:183)
at org.eclipse.core.internal.resources.Folder.assertCreateRequirements(Folder.java:43)
at org.eclipse.core.internal.resources.Folder.create(Folder.java:101)
at org.eclipse.core.internal.resources.Folder.create(Folder.java:129)
at org.eclipse.jdt.internal.ui.util.ResourcesUtility.createFolder(ResourcesUtility.java:77)
at org.eclipse.jdt.internal.ui.util.CoreUtility.createFolder(CoreUtility.java:64)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:732)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:710)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:696)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:683)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:671)
at org.eclipse.jdt.ui.tests.quickfix.UtilitiesTest.setUp(UtilitiesTest.java:74)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
Caused by: java.lang.Exception: Resource '/TestSetupProject' does not exist.
at org.eclipse.core.internal.resources.ResourceException.provideStackTrace(ResourceException.java:42)
at org.eclipse.core.internal.resources.ResourceException.(ResourceException.java:38)
... 15 more
Suppressed: Java Model Exception: Error in Java Model (code 969): TestSetupProject does not exist
at org.eclipse.jdt.internal.core.JavaElement.newNotPresentException(JavaElement.java:546)
at org.eclipse.jdt.internal.core.JavaModelManager.getPerProjectInfoCheckExistence(JavaModelManager.java:2535)
at org.eclipse.jdt.internal.core.JavaProject.getPerProjectInfo(JavaProject.java:2329)
at org.eclipse.jdt.internal.core.JavaProject.getOutputLocation(JavaProject.java:2144)
at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3512)
at org.eclipse.jdt.testplugin.JavaProjectHelper.lambda$1(JavaProjectHelper.java:604)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2457)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2477)
at org.eclipse.jdt.testplugin.JavaProjectHelper.clear(JavaProjectHelper.java:612)
at org.eclipse.jdt.ui.tests.quickfix.UtilitiesTest.tearDown(UtilitiesTest.java:79)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
```

I could not find a special error in the logfiles that would be the root cause. But there is a IllegalArgumentException logged that i could not reproduce locally:

https://download.eclipse.org/eclipse/downloads/drops4/I20241217-1800/testresults/ep435I-unit-win32-x86_64-java21_win32.win32.x86_64_21/org.eclipse.jdt.ui.tests.AutomatedSuite.txt

```
java.lang.IllegalArgumentException:
at org.eclipse.core.runtime.Assert.isLegal(Assert.java:68)
at org.eclipse.core.runtime.Assert.isLegal(Assert.java:52)
at org.eclipse.jdt.internal.ui.workingsets.WorkingSetModel.setWorkingSets(WorkingSetModel.java:429)
at org.eclipse.jdt.internal.ui.workingsets.WorkingSetModel.workingSetManagerChanged(WorkingSetModel.java:616)
at org.eclipse.ui.internal.AbstractWorkingSetManager$5.run(AbstractWorkingSetManager.java:378)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
at org.eclipse.ui.internal.AbstractWorkingSetManager.lambda$1(AbstractWorkingSetManager.java:386)
at org.eclipse.ui.internal.AbstractWorkingSetManager.firePropertyChange(AbstractWorkingSetManager.java:391)
at org.eclipse.ui.internal.AbstractWorkingSetManager.internalAddWorkingSet(AbstractWorkingSetManager.java:235)
at org.eclipse.ui.internal.AbstractWorkingSetManager.addWorkingSet(AbstractWorkingSetManager.java:227)
at org.eclipse.ui.internal.WorkingSetManager.addWorkingSet(WorkingSetManager.java:66)
at org.eclipse.jdt.ui.tests.packageview.WorkingSetDropAdapterTest.createJavaWorkingSets(WorkingSetDropAdapterTest.java:185)
at org.eclipse.jdt.ui.tests.packageview.WorkingSetDropAdapterTest.testRearrange2(WorkingSetDropAdapterTest.java:193)
```

However that IllegalArgumentException happened on other build too. like
https://download.eclipse.org/eclipse/downloads/drops4/I20241217-1800/testresults/ep435I-unit-linux-x86_64-java21_linux.gtk.x86_64_21/org.eclipse.jdt.ui.tests.AutomatedSuite.txt

Start with WorkingSetModel.setWorkingSets and workingSetManagerChanged, then inspect WorkingSetDropAdapterTest.testRearrange2 and createJavaWorkingSets for the reported exception. Compare those failures with UtilitiesTest.setUp and tearDown, JavaProjectHelper.addSourceContainer, and JavaProjectHelper.clear. Done means identifying the cause of the cross-platform test failures and documenting or fixing it so the affected tests no longer fail.

Written by the indexing model from the issue text.