Why does calling CheckSynchronize in an Android environment crash

[a8265348]

code:
class procedure TThreadSynchronisationHandler.Update(Sender: TObject);
  begin
    CheckSynchronize(); //This has to be done in the main thread regularly to enable Synchronisation between threads.
  end;

  class procedure TThreadSynchronisationHandler.Initialise;
  begin
    if not IsInitialised then
    begin
      ApplicationProperties.OnUpdate.Add({$ifdef FPC}@{$endif}Update);
      IsInitialised := true;
    end;
  end;

procedure TCastleTCPClientThread.Execute;
  var
    LMessage: String;
  begin
    FClient.Connect;
    FClient.IOHandler.ReadTimeout := 100;

    if Assigned(FOnConnected) then
      Queue(FOnConnected);

    while not Terminated do
    begin
      try
        LMessage := FClient.IOHandler.ReadLn; //Result is empty when timeout reached.

        if LMessage <> '' then
        begin
          FMessageList.Add(LMessage);
          Queue(FOnMessageReceived);
        end;
      except
        //Exception occurs when connection was disconnected (gracefully).
        Terminate;
        if Assigned(FOnDisconnected) then
          Synchronize(FOnDisconnected); //Must be synchronize, not queue, because queued messages get lost.
      end;
    end;
    FClient.Disconnect;
    if FreeClientOnTerminate then
      FClient.Free;
  end;