[Editor] `MaterialsVariantsComponent`'s dropdown doesn't initialize to the current value

[anonymous2585]

Describe the bug
When using a MaterialsVariantsComponent, unselecting the GameObject and re-selecting it clears the value displayed in the inspector.

To Reproduce
Steps to reproduce the behavior:

1. Add a Multiple Instance script on a GameObject, with url set to a glTF file using the "KHR_materials_variants" extension and Quantity set to 1.
2. Press Play
3. Select the created object "glTF-0"
4. In the Materials Variants Component, change the dropdown to one of the variant of your model
5. Unselect the GameObject
6. Re-select the GameObject
7. See MaterialsVariantsComponent displaying <no variant>

Expected behavior
The variant dropdown displays the variant selected at step 5.

Desktop (please complete the following information):

• glTFast version 6.18.0
• Unity Editor version 6000.3.12f1
• Platform: Editor

Note: I didn't update the [Unreleased] section of the changelog since the main branch here is not up to date with version 6.18.0

[atteneder]

Thanks for reporting and fixing. I will merge this internally soon. Continuing conversation in #48 .