How to reproduce:
1. Open the attached "1243035 repro" project
2. Select "NewImage.png" from the Assets > AdvancedTextureImporter > ExampleTexture folder

Expected result: no errors are thrown
Actual result: errors stating that the previous instance hasn't been disposed correctly are thrown

Reproducible with: 2019.2.0a13, 2019.2.21f1, 2019.4.0f1, 2020.1.0b13, 2020.2.0a15
Not reproducible with: 2018.4.23f1, 2019.2.0a12

Notes:
-On affected versions the bug doesn't necessarily reproduce on the first selection, deselect and repeat the 2nd step a few times to reproduce.
-Very rarely the Editor crashes after selecting the "NewImage.png"
-The Editor sometimes crashes when recompiling the "TextureImporterEditor.cs" Script
-The full error states: The previous instance of UnityEditor.TextureImporterInspector has not been disposed correctly. Make sure you are calling base.OnDisable() in your AssetImporterEditor implementation.
UnityEditor.Editor:CreateEditor (UnityEngine.Object[],System.Type)
AdvancedTextureImporter.TextureImporterEditor:OnEnable () (at Assets/AdvancedTextureImporter/Scripts/Editors/TextureImporterEditor.cs:22)