For NMS code:

     - WorldServer.class:
         public TileEntity getTileEntity(BlockPosition pos)
          > TileEntityStructure is missing as a possible variant of TileEntity to be returned.

This issue causes CraftStructureBlock to error upon:

(TileEntityStructure)((CraftWorld)block.getWorld()).getTileEntityAt(this.getX(), this.getY(), this.getZ());

yielding a ClassCastException if the original block was a type that possessed a TileEntity...

Example:

  > For Block a: a's Material (type) is Material.ENDER_CHEST
  > a.setType(Material.STRUCTURE_BLOCK);

Now attempting to call getTileEntityAt will still yield an instance of TileEntityEnderChest, and attempting to use block.getState() will produce a ClassCastException.

If a was a type not associated with a TileEntity (ie. Material.WOOL), setting a to type Material.STRUCTURE_BLOCK will yield the appropriate TileEntity as an instance of TileEntityStructure.

 

"Caused by: java.lang.ClassCastException: net.minecraft.server.v1_12_R1.TileEntityEnderChest cannot be cast to net.minecraft.server.v1_12_R1.TileEntityStructure"