Fill command broken (null1 minecraft:yellow_concretenull)

When I use the "create text with" block on an item, for instance yellow concrete, it turns it into "null1 yellow_concretenull"

Is there any way I can fix this? I need to make a fill command with two variable points and a variable block to fill with. I have figured out the corners part but the block is being annoying.

I'm making it broadcast the command to the chat so I can look for errors and then try to run the command. The command does nothing obviously since it's broken. I've tried using "Replace (null) with () on (item)" and "Substring (item) from position (5) to position ((length of (item)) - 4)

Neither of them work.