Cross skeleton reference - Clipping End slot

[HaraldCsaszar]

A clipping area of a first skeleton can have another skeleton's slot set as End slot reference. Since separate json files are exported per skeleton, these cross-skeleton references will lead to errors upon import.

Reported on the forum, repro files and description:
http://esotericsoftware.com/forum/Error-reading-skeleton-JSON-file-for-SkeletonData-asset-11629

Steps to reproduce:

1. Download project package from here: https://www.dropbox.com/s/zq1k7lvatm0l49c/TyphonFiles.zip?dl=0. Unzip to TyphonFiles.
2. Open project TyphonFiles/Project/TyphonTEST.spine in Spine editor.
3. Filter for clipping, select element named rootin first skeleton Typhoon 1x2.
4. Notice that End slot is set to appear_00. This slot belongs to second skeleton Typhoon 1x3. No error message is provided although this is an illegal reference.

Note that in the current version 3.8. 03-beta of Spine a slot of another skeleton cannot be selected, it is greyed out.

Was first reported as ticket
EsotericSoftware/spine-runtimes#1308

[NathanSweet]

3.7.92 projects with this problem are fixed on load.