assertIsWriteThread fails when calling copyDirectoryToProject

I want to write a unit test for my plugin. The plugin retrieves information from git, so I want a .git directory in the test project. I have created a directory `testData/single/dot_git` in my plugin's directory, and I have the following test:

public class TestGitLink extends BasePlatformTestCase {
    @Override
    @BeforeEach
    protected void setUp() throws Exception {
        super.setUp();
        this.myFixture.copyDirectoryToProject("dot_git", ".git");
    }

    @Override
    @AfterEach
    protected void tearDown() throws Exception {
        super.tearDown();
    }

    @Test
    public void testSomethign() {
        assertEquals("", getTestDataPath());
    }

    @Override
    protected String getTestDataPath() {
        return "testData/single/";
    }
}

When I run this test, it fails with the following error:

com.intellij.openapi.diagnostic.RuntimeExceptionWithAttachments: EventQueue.isDispatchThread()=false Toolkit.getEventQueue()=com.intellij.ide.IdeEventQueue@51f9a9a4
Current thread: Thread[Test worker,5,main] 778739793
Write thread (volatile): Thread[AWT-EventQueue-0,6,main] 329367088com.intellij.openapi.diagnostic.Attachment@669a08db
	at com.intellij.openapi.application.impl.ApplicationImpl.assertIsWriteThread(ApplicationImpl.java:1058)
	at com.intellij.openapi.application.impl.ApplicationImpl.startWrite(ApplicationImpl.java:1140)
	at com.intellij.openapi.application.impl.ApplicationImpl.runWriteActionWithClass(ApplicationImpl.java:945)
	at com.intellij.openapi.application.impl.ApplicationImpl.runWriteAction(ApplicationImpl.java:968)
	at com.intellij.testFramework.fixtures.impl.LightTempDirTestFixtureImpl.copyAll(LightTempDirTestFixtureImpl.java:81)
	at com.intellij.testFramework.fixtures.impl.LightTempDirTestFixtureImpl.copyAll(LightTempDirTestFixtureImpl.java:76)
	at com.intellij.testFramework.fixtures.impl.CodeInsightTestFixtureImpl.copyDirectoryToProject(CodeInsightTestFixtureImpl.java:449)

The similar `copyFileToProject` does not raise an exception. My test runs as expected if I don't call `copyDirectoryToProject`. What is going on? Do I have to do something before I copy a directory?