Steps: 

1. create a new world with CHEATS ON.
2. run /give @p structure_block
3. place the StructureBlock on the ground.
4. place a button next to it.
5. click on the StructureBlock.
6. change Offset Y=1
7. change Mode to "Load"
8. set Structure Name >> a_random_text
9. exit StructureBlock menu.
10. click on the button(placed on step:4).
11. click on the StructureBlock.

Observed results: 

the button "Load" is enabled.

Expected results: 

the button "Load" shouldn't be enabled, because the structure name doesn't exist.