"null instance" is what you get from
get_node() when you give it an invalid path.
If you're getting this error, it means that you are not using a valid path to get the node you want. It's impossible to tell you exactly what you're doing wrong because you haven't showed your node setup or where in your code you're trying to access the node. Regardless, you must give the correct node path in order to get the node.
It's important to understand how node paths work. If you're familiar with the directory structure of your operating system (Windows, MacOS, Linux), they work in exactly the same way.
This link may help you with understanding how to get a node: