WebSocket throws error

[SaharMehrpour]

It sometimes stops working.
The error in IntelliJ

org.java_websocket.exceptions.WebsocketNotConnectedException
	at org.java_websocket.WebSocketImpl.send(WebSocketImpl.java:630)
	at org.java_websocket.WebSocketImpl.send(WebSocketImpl.java:607)
	at ChatServer.sendToAll(ChatServer.java:100)
	at FileChangeManager.sendMessage(FileChangeManager.java:358)
	at FileChangeManager$2.selectionChanged(FileChangeManager.java:114)
	at com.intellij.util.messages.impl.MessageBusImpl.invokeMethod(MessageBusImpl.java:674)
	at com.intellij.util.messages.impl.MessageBusImpl.invokeListener(MessageBusImpl.java:649)
	at com.intellij.util.messages.impl.MessageBusImpl.deliverMessage(MessageBusImpl.java:422)
	at com.intellij.util.messages.impl.MessageBusImpl.pumpWaitingBuses(MessageBusImpl.java:397)
	at com.intellij.util.messages.impl.MessageBusImpl.pumpMessages(MessageBusImpl.java:379)
	at com.intellij.util.messages.impl.MessageBusImpl.access$100(MessageBusImpl.java:33)
	at com.intellij.util.messages.impl.MessageBusImpl$MessagePublisher.invoke(MessageBusImpl.java:185)
	at com.sun.proxy.$Proxy112.selectionChanged(Unknown Source)
	at com.intellij.openapi.fileEditor.impl.FileEditorManagerImpl.lambda$fireSelectionChanged$29(FileEditorManagerImpl.java:1672)
	at com.intellij.openapi.fileEditor.impl.FileEditorManagerImpl$8.lambda$run$0(FileEditorManagerImpl.java:1150)
	at com.intellij.openapi.util.ExpirableRunnable$1.run(ExpirableRunnable.java:17)
	at com.intellij.openapi.wm.impl.FocusManagerImpl.lambda$doWhenFocusSettlesDown$3(FocusManagerImpl.java:182)
	at com.intellij.util.ui.EdtInvocationManager.invokeLaterIfNeeded(EdtInvocationManager.java:101)
	at com.intellij.ide.IdeEventQueue.ifFocusEventsInTheQueue(IdeEventQueue.java:188)
	at com.intellij.ide.IdeEventQueue.executeWhenAllFocusEventsLeftTheQueue(IdeEventQueue.java:140)
	at com.intellij.openapi.wm.impl.FocusManagerImpl.doWhenFocusSettlesDown(FocusManagerImpl.java:173)
	at com.intellij.openapi.wm.impl.FocusManagerImpl.doWhenFocusSettlesDown(FocusManagerImpl.java:179)
	at com.intellij.openapi.wm.impl.FocusManagerImpl.lambda$doWhenFocusSettlesDown$2(FocusManagerImpl.java:187)
	at com.intellij.openapi.application.TransactionGuardImpl.runWithWritingAllowed(TransactionGuardImpl.java:218)
	at com.intellij.openapi.application.TransactionGuardImpl.access$200(TransactionGuardImpl.java:21)
	at com.intellij.openapi.application.TransactionGuardImpl$2.run(TransactionGuardImpl.java:200)
	at com.intellij.openapi.application.impl.ApplicationImpl.runIntendedWriteActionOnCurrentThread(ApplicationImpl.java:781)
	at com.intellij.openapi.application.impl.ApplicationImpl.lambda$invokeLater$4(ApplicationImpl.java:319)
	at com.intellij.openapi.application.impl.FlushQueue.doRun(FlushQueue.java:84)
	at com.intellij.openapi.application.impl.FlushQueue.runNextEvent(FlushQueue.java:133)
	at com.intellij.openapi.application.impl.FlushQueue.flushNow(FlushQueue.java:46)
	at com.intellij.openapi.application.impl.FlushQueue$FlushNow.run(FlushQueue.java:189)
	at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:313)
	at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:776)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:727)
	at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
	at java.base/java.security.AccessController.doPrivileged(Native Method)
	at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
	at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:746)
	at com.intellij.ide.IdeEventQueue.defaultDispatchEvent(IdeEventQueue.java:969)
	at com.intellij.ide.IdeEventQueue._dispatchEvent(IdeEventQueue.java:839)
	at com.intellij.ide.IdeEventQueue.lambda$dispatchEvent$8(IdeEventQueue.java:449)
	at com.intellij.openapi.progress.impl.CoreProgressManager.computePrioritized(CoreProgressManager.java:808)
	at com.intellij.ide.IdeEventQueue.lambda$dispatchEvent$9(IdeEventQueue.java:448)
	at com.intellij.openapi.application.impl.ApplicationImpl.runIntendedWriteActionOnCurrentThread(ApplicationImpl.java:781)
	at com.intellij.ide.IdeEventQueue.dispatchEvent(IdeEventQueue.java:496)
	at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
	at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
	at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
	at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)