Resolve generic type from AST into actual type

The problem is about getting the actual type of function parameter to check it with checker.isNullableType. I should find it from the function call statement, i.e. I can't select the string | null statement directly.

type Action1<T> = (arg1: T) => void;
const fn: Action1<string | null> = (arg1: string | null): void => {};
fn(null);

https://ts-ast-viewer.com/#code/C4TwDgpgBAggxsAlgewHYEYA8AVAfFAXigAoBDAJwHN0AuKbASkPwDdlEATAbgCg40AzsCgAzVHXhI0WIeUSpKUAD5RUAVwA2G-ETJVaUWfMUr1WhnTadmUAN4BfXmOJmNDLkA

Dev tools console:

var fnCallNode = sourceFile.statements[2].getChildAt(0).getChildAt(0);
var fnType = checker.getTypeAtLocation(fnCallNode);
console.log(checker.typeToString(fnType)); // "Action1<string | null>"

var parameterTypeDeclaration = fnType.getCallSignatures()[0].getParameters()[0].declarations[0];
var parameterType = checker.getTypeAtLocation(parameterTypeDeclaration);
console.log(checker.typeToString(parameterType)); // "T"
console.log(checker.isNullableType(parameterType)); // false

^ The desired result is to get the type "string | null" for which checker.isNullable is true. I have no idea how to expand generic type into the actual one.

It turned out that I should start the search from the whole fn(null) statement and use checker.getResolvedSignature:

var fnCallNode = sourceFile.statements[2].getChildAt(0); // "fn(null)"
var fnCallSignature = checker.getResolvedSignature(fnCallNode);
var parameter = fnCallSignature.getParameters()[0];
var parameterType = checker.getTypeOfSymbolAtLocation(parameter, parameter.declarations[0]);

console.log(checker.typeToString(parameterType)); // "string | null"
console.log(checker.isNullableType(parameterType)); // true