Hello! Just wanted to say I appreciate everyone's responses and support!
in advance :).
Currently, I'm working on an inventory system and everything seems to be working beside one little error.
The debugger says: "Attempt to call function 'set_texture' in base 'null instance' on a null instance."
Which is because on line 5 I get an error for get_node: (Node not found: "Background/Inv1/TextureRect" (relative to "root/Control").)
What I'm trying to accomplish.
For every item in the player's inventory or inv_data, it places the image of that item in the appropriate inventory slot
I don't understand because if you look in my screenshot the root or "Control" node has Background/Inv1/TextureRect under it. Can someone explain why I'm getting an error and how I can fix it? 
Thanks!