TMP Font Asset Creator throws errors when saving a generated Font Asset for a built-in/package source font

Steps to reproduce:

1.  Create new Unity project
2. Open TMP Font Asset Creator window (Window > TextMeshPro > Font Asset Creator)
3. Import TMP Essentials
4.  In Source Font select any built-in font (e.g. Roboto-Bold)
5.  Press "Generate Font Atlas" button
6.  When generated, press "Save as..." or "Save"
7. Observe the opened directory
8. Save the asset
9. Observe the Console

Actual results: "Couldn't create asset file!", "GUI Error: Invalid GUILayout state..." and "UnityException: Creating asset at path..." errors are thrown because the save dialog defaults to a path inside the Library folder

Expected results: The save dialog defaults to a writable location inside the Assets folder, and no errors are thrown

Reproducible with versions: 6000.0.0f1, 6000.3.21f1, 6000.5.7f1, 6000.6.0b8, 6000.7.0a4

Tested on (OS): Windows

Notes:

• Same issue was fixed for TextCore [Internal link]