Steps to reproduce: 

1. Create a flat world Cheats ON.
2. Run /give @p structure_block
3. Place a StructureBlocks on the ground.
4. Place a Stone button next to it.
5. Place some blocks inside the StructureBlock area.
6. Set "Save to Internal Memory"(default)

Observed results: 

Using "Save" button from the StructureBlock Menu will save the structure to disk.

Expected results: 

StructureBlock should save the structure to internal memory.

NOTE:  

When The Stone button is used, the structure is saved to Internal Memory.