Cannot save PersistentStateComponent when class has project even if project is private

PersistentStateComponent cause Error on save when the class has private project.

The error message is like this.

Unable to serialize TestState state

com.intellij.util.xmlb.XmlSerializationException: Can't serialize instance of class models.TestState
 at com.intellij.configurationStore.XmlSerializer.serialize(xmlSerializer.kt:59)
 at com.intellij.configurationStore.XmlSerializer.serialize$default(xmlSerializer.kt:43)
 at com.intellij.configurationStore.SaveSessionBaseKt.serializeState(SaveSessionBase.kt:57)
 at com.intellij.configurationStore.SaveSessionBase.setState(SaveSessionBase.kt:29)
 at com.intellij.configurationStore.StateStorageManagerImpl$startExternalization$1.setState(StateStorageManagerImpl.kt:440)
 at com.intellij.configurationStore.ComponentStoreImpl.commitComponent(ComponentStoreImpl.kt:251)
 at com.intellij.configurationStore.ComponentStoreImpl.doSaveComponents(ComponentStoreImpl.kt:198)
 at com.intellij.configurationStore.ComponentStoreImpl.save(ComponentStoreImpl.kt:134)
 at com.intellij.openapi.components.impl.stores.StoreUtil.save(StoreUtil.java:40)
 at com.intellij.openapi.project.impl.ProjectImpl.save(ProjectImpl.java:331)
 at com.intellij.openapi.project.impl.ProjectImpl.save(ProjectImpl.java:315)
 at com.intellij.openapi.components.impl.stores.StoreUtil.saveProject(StoreUtil.java:137)
 at com.intellij.openapi.components.impl.stores.StoreUtil.saveProjectsAndApp(StoreUtil.java:128)
 at com.intellij.openapi.components.impl.stores.StoreUtil.saveDocumentsAndProjectsAndApp(StoreUtil.java:113)
 at com.intellij.openapi.application.impl.ApplicationImpl.saveAll(ApplicationImpl.java:1441)
 at com.intellij.ide.SaveAndSyncHandlerImpl.saveProjectsAndDocuments(SaveAndSyncHandlerImpl.java:126)
 at com.intellij.ide.SaveAndSyncHandlerImpl$2.lambda$onFrameDeactivated$0(SaveAndSyncHandlerImpl.java:94)
 at com.intellij.openapi.application.TransactionGuardImpl.runSyncTransaction(TransactionGuardImpl.java:88)
 at com.intellij.openapi.application.TransactionGuardImpl.lambda$submitTransaction$1(TransactionGuardImpl.java:111)
 at com.intellij.openapi.application.TransactionGuardImpl.submitTransaction(TransactionGuardImpl.java:120)
 at com.intellij.openapi.application.TransactionGuard.submitTransaction(TransactionGuard.java:122)
 at com.intellij.ide.SaveAndSyncHandlerImpl$2.onFrameDeactivated(SaveAndSyncHandlerImpl.java:92)
 at com.intellij.ide.FrameStateManagerImpl.fireDeactivationEvent(FrameStateManagerImpl.java:86)
 at com.intellij.ide.FrameStateManagerImpl.access$500(FrameStateManagerImpl.java:32)
 at com.intellij.ide.FrameStateManagerImpl$2.lambda$applicationDeactivated$0(FrameStateManagerImpl.java:72)
 at com.intellij.util.concurrency.QueueProcessor.runSafely(QueueProcessor.java:232)
 at com.intellij.util.Alarm$Request.runSafely(Alarm.java:405)
 at com.intellij.util.Alarm$Request.access$700(Alarm.java:330)
 at com.intellij.util.Alarm$Request$1.run(Alarm.java:371)
 at com.intellij.openapi.application.TransactionGuardImpl$2.run(TransactionGuardImpl.java:315)
 at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.doRun(LaterInvocator.java:447)
 at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.runNextEvent(LaterInvocator.java:431)
 at com.intellij.openapi.application.impl.LaterInvocator$FlushQueue.run(LaterInvocator.java:415)
 at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
 at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:762)
 at java.awt.EventQueue.access$500(EventQueue.java:98)
 at java.awt.EventQueue$3.run(EventQueue.java:715)
 at java.awt.EventQueue$3.run(EventQueue.java:709)
 at java.security.AccessController.doPrivileged(Native Method)
 at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
 at java.awt.EventQueue.dispatchEvent(EventQueue.java:732)
 at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.java:781)
 at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.java:722)
 at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:382)
 at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
 at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
 at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
 at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
 at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
Caused by: java.lang.RuntimeException: java.lang.NoSuchMethodException: models.TestState.<init>()
 at com.intellij.util.ExceptionUtil.rethrow(ExceptionUtil.java:160)
 at com.intellij.util.ReflectionUtil.newInstance(ReflectionUtil.java:427)
 at com.intellij.util.xmlb.SkipDefaultValuesSerializationFilters.getDefaultBean(SkipDefaultValuesSerializationFilters.java:59)
 at com.intellij.util.xmlb.SkipDefaultsSerializationFilter.equal(SkipDefaultsSerializationFilter.java:25)
 at com.intellij.util.xmlb.BeanBinding.serializeInto(BeanBinding.java:83)
 at com.intellij.util.xmlb.BeanBinding.serialize(BeanBinding.java:68)
 at com.intellij.configurationStore.XmlSerializer.serialize(xmlSerializer.kt:49)
 ... 49 more

Class implementation is like this.

@State(
 name = "TestState"
)
public class TestState implements PersistentStateComponent<TestState> {
 private Project project;
 public String test;

 public static TestState getInstance(Project project) {
 return ServiceManager.getService(project, TestState.class);
 }

 public TestState(Project project) {
 this.project = project;
 }

 public void update(List<PullRequest> pullRequests) {
 this.test = "test";
 }

 @Nullable
 @Override
 public TestState getState() {
 return this;
 }

 @Override
 public void loadState(@NotNull TestState state) {
 XmlSerializerUtil.copyBean(state, this);
 }
}

Private variables is not saved. Why it cause error??

Do you have any annotation or something to prevent this error?