How do you fix cannot read property 'toLowerCase' of null?

[Woz]

I ran into this problem, and had tried to find solution online, but they all say things like codes and stuffs that i couldnt understand, anyone can help or at least a simpler explanation please. Thank you

 

[Damp Toast]

Somewhere in your code you are using a method 'toLowerCase'. The variable you are calling it from is set to null instead of a value for whatever reason.