Steps to reproduce:
1. Download and open the attached project
2. Go to Window/Pixel Terrain/Import Texture and import a png file
3. Set it as a texture in the TerrainGenerator script
4. Press Analyze

Expected result: colors from texture are extracted and displayed in the inspector
Actual result: texture remains unreadable and an error is thrown

Reproduced on 5.5.4p2, 5.6.3f1, 2017.1.0p2, 2017.2.0b6, 2017.3.0a3

Notes:
-The texture appears to have the flag checked in the inspector until the Editor is restarted
-Setting the flag manually works as expected