Upload>Model button is sometimes disabled on MacOS

[AtlasLinden]

Environment

Second Life Project 7.1.14.15976006598 (GLTF Mesh Import)

Description

The Upload>Model button is sometimes disabled on MacOS.

Reproduction steps

Further investigation is required on what can cause this.
It appears to occur after uploading multiple models and performing operations with Build tools. Possibly after deleting rezzed meshes.

[akleshchev]

Tweaking this logic a bit, hopefully it will help.