If you change Windows sizes, (i.e. going from Fullscreen to Windowed, etc) and you're in the command block GUI, the command disappears and all the buttons are disabled except for the cancel button. If you close the GUI and reopen it, everything is fine.
Environment
Windows 10 64-Bit
Java 8 Update 102
Processor: 4x AMD A6-5200
Graphics: AMD Radeon HD 8400
Dupe of MC-258