Signal name can not be detected during update diagram

Hello there!

I'm having a model with quite a number of subsystems. During update diagram some block callbacks are running which determine the signal names of the input ports by

 LineHandles = get_param(block,'LineHandles')
 PortHandle = get_param(LineHandles.(PortNum),'SrcPortHandle')
 PropagatedSignals = get_param(PortHandle,'Name')

Anyway, most of the signals can be detected, but some are not (always in the same subsystems). So I get an error after model update :-(. What I did then was

 open_system(get_param(block,'Parent'),'tab')
 get_signal_name(...) %as described above
 close_system(get_param(block,'Parent'))

This works a little bit better because after every model update one signal name more is found. So I can repeat this procedure as long as every signal name is found, but obviously this is not very satisfying. So my question is what happens here and what can I do against it? I could ask if the signal name is empty, but after that I should stop updating (I guess there is no possibility to do so...) and update again. But actually this behaviour should not be as it is I think. Thanks for your help! Greetings

Matthias