output: #fixed code
                                @Test
    public void definition_callee_symbol() {
      FileInput tree = parse(
        "def fn(): pass",
        "fn('foo')"
      );
      assertNameAndQualifiedName(tree, "fn", null);
    }
input: #vulnerable code 
                                @Test
    public void definition_callee_symbol() {
      FileInput tree = parse(
        "def fn(): pass",
        "fn('foo')"
      );
      // TODO: create a symbol for function declaration
      CallExpression callExpression = getCallExpression(tree);
      assertThat(callExpression.calleeSymbol()).isNull();
    } 
                              #location 9 
                              #vulnerability type NULL_DEREFERENCE