Fix bug where an undefined arrival airport will cause plugin to crash

[arthuwu]

(I am 99% sure this is why, but I don't currently have access to a computer with VS installed and the WiFi at my hotel is not exactly fast so I haven't been able to test it yet. Will try to install VS on my laptop tonight but if I'm not able to can someone pretty please test it for me uwu 👉👈)
ppdate: installed VS, tested, it works and i am stupid

Testing instructions

• start a sweatbox session
• use a tag with the APPDEPLine4 or AMCLine4 element
• assume and expand its tag
• create a fpl for an aircraft from departing VHHH and not landing at VHHH, VHHX or VMMC
• assign it a SID
• append [STAR]/[RUNWAY] to the flight plan
• switch between the tagged(expanded) and untagged states
• click the line 4 element; it should not open the approach selection menu

expected result:

• line 4 should display the SID once it has been assigned, regardless of whether a STAR was added to the flight plan
• the approach selection menu does not open when line 4 is clicked