fix crash of dictionary.subscript.getter for closed groups

SessionMessagingKit/Sending & Receiving/Pollers/ClosedGroupPoller.swift

@@ -44,7 +44,7 @@ public final class ClosedGroupPoller : NSObject {
         // Might be a race condition that the setUpPolling finishes too soon,
         // and the timer is not created, if we mark the group as is polling
         // after setUpPolling. So the poller may not work, thus misses messages.
-        isPolling[groupPublicKey] = true
+        internalQueue.sync{ isPolling[groupPublicKey] = true }
         setUpPolling(for: groupPublicKey)
     }
 
@@ -55,7 +55,7 @@ public final class ClosedGroupPoller : NSObject {
     }
 
     public func stopPolling(for groupPublicKey: String) {
-        isPolling[groupPublicKey] = false
+        internalQueue.sync{ isPolling[groupPublicKey] = false }
         timers[groupPublicKey]?.invalidate()
     }