Fix relations drawn on non existent notes

[Crowerade]

Addresses #379

[yrammos]

@Crowerade I'm afraid that even after a580a98 and a4f9eab (i.e. the last two commits), creating a meta-relation with relations that are reduced out in other layers still causes a crash. I tested with minimal_analysis_2.mei.

[Crowerade]

I tried reproducing it, but it worked fine. Could you tell what steps you followed to have the crash?

[yrammos]

@Crowerade to reproduce, perhaps you need to create a meta-relation with two relations, only one of which has been reduced-out in the second layer. The other relation in this test existed in both layers.

(I repeated the test again just now to be sure.)

[Crowerade]

That's curious, it does not emit any error

[yrammos]

@Crowerade I sent you a screencap via Mattermost reproducing the bug.

[Crowerade]

This should be fixed

[yrammos]

@Crowerade this is good, thanks.

Unfortunately our predicament with layers continue: un-reducing the added layer (thus adding back the previously removed notes) fails to show the new relations and meta-relations that involved them.

Sent you a video on Mattermost to clarify.

[yrammos]

Essentially, whenever we un-reduce, after putting back all relevant notes we should have a second step to add any graph (meta-)relations that became renderable.

[Crowerade]

I'll look into that

[Crowerade]

This is not planned in the end since we want to have static layers instead of dynamic and updatable ones