Command deletes after closing GUI

After typing a command into a command block, the text will sometimes delete itself after I press the X in the top right corner, and the GUI will remain open. If you press cancel, then the X (should this glitch occur) the text will stay active, but will not reappear in the GUI until it has been reopened. If you press the X until the GUI closes, the text will be completely lost.

EDIT: The text simply disappears from the GUI. It will remain active in the Command Block, it only disappears from the text field.