This bug in OSX 10.11 (El Capitan)'s Audio MIDI Setup application looks nasty but in reality it is pretty harmless, at least in our tests. However since it results in Audio MIDI Setup crash it may lead you to believe that the changes you just made did not "take". Fortunately this is not the case.
The bug itself causes the Audio MIDI Setup to crash whenever the MIDI Network Setup window is closed, or if you close the main MIDI Studio window while the MIDI Network Setup window is still open. The MIDI Network Setup window is used to create and configure network MIDI sessions that are used by devices that wish to send and receive MIDI wirelessly, for example between an iPad and a Mac. Normally you only set up a network MIDI session once as CoreMIDI remembers the state of all your defined sessions, whether they are enabled or not, and starts the enabled ones up whenever you log into OSX after your restart your Mac. Most users rarely use the MIDI Network Setup portion of Audio MIDI Setup so this bug is not one you will encounter very often.
That said, it can be disconcerting, and since it is a sudden termination you may be mislead into thinking that the changes you just made may be lost. This does not appear to be the case. So the bug is more nuisance than anything.
However, if you are uncomfortable about Audio MIDI Setup crashing right after you have just created a new network MIDI session, or made changes to existing ones, then I recommend that you simply quit the application without closing the MIDI Network Setup window. Audio MIDI Setup will exit without crashing. Note though that the next time you start Audio MIDI Setup the window will show. If that is a problem close it before you make any changes. This will cause the crash but since no data has changed there should be little chance for data loss. The next time you run Audio MIDI Setup the window will not appear.