I might have found a bug in the latest daily build (B2 daily Sep 18 2018 (core 3 (DailyBuild Sep 18 2018))
When you open the Material Node System window at the same time as the Texture Manager window, you're not able to select a texture (to relink for example) in the Texture Manager window.
It flashes for a second and deselect itself.
As soon as the Material Node System window is closed, you can select again in the Texture Manager.
Tell me if you can reproduce it or if you need more information, thank you!