Summary:
The Minecart with Command Block edit interface does not retain previously filled fields.

Steps to Reproduce:

1. In Creative, obtain a Minecart with Command Block using the command:
   /give @s command_block_minecart
2. Place it down on a rail.
3. Open the interface to edit the command block (long press or click Open).
4. Fill in Command Input, and other options eg. Delay in Ticks.
5. Close interface (sets the command).
6. Pushing/sending the minecart over a powered Activator Rail will trigger the command.
7. Re-opening the interface shows all the fields blank.
8. Closing the interface again without making any new changes will not erase the command. It will still trigger correctly.
9. Changing any other field eg. Hover Note, Execute on First Tick, Delay in Ticks, or Previous Output and then closing the interface will erase the saved Command Input.
10. Pushing/sending the minecart over a powered Activator Rail will not trigger any command.

Observed Results:
Open Minecart with Command Block interface input a command, and close it.
Re-open Minecart with Command Block interface shows blank fields.

Expected Results:
Re-opening the Command Block interface should show the previously saved values. 

Screenshots/Videos attached: Yes