c# - SyntaxWalker Visit method -

i have problem class syntaxwalker , method:

visitmemberaccessexpression(memberaccessexpressionsyntax node) 

i node type , if equal example: "person" add list. use:

if(semanticmodel.gettypeinfo(node).type.name == modelname) list.add("stack"); 

but have error: "object reference not set instance of object" why? fields:

private isemanticmodel semanticmodel; public string modelname { get; private set; } 

set model name:

modelname = type.basetype.typearguments[0].name.tostring(); 

i've been on few days , not know please please. in accordance advice: how define "node" type in roslyn?