fix(chat) Only display private message dialog for active participants (#13708)

react/features/chat/middleware.ts

@@ -130,7 +130,10 @@ MiddlewareRegistry.register(store => next => action => {
             // recipient. This logic tries to mitigate this risk.
             const shouldSendPrivateMessageTo = _shouldSendPrivateMessageTo(state, action);
 
-            if (shouldSendPrivateMessageTo) {
+            const participantExists = shouldSendPrivateMessageTo
+                && getParticipantById(state, shouldSendPrivateMessageTo);
+
+            if (shouldSendPrivateMessageTo && participantExists) {
                 dispatch(openDialog(ChatPrivacyDialog, {
                     message: action.message,
                     participantID: shouldSendPrivateMessageTo