#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
#fixed code
                              @Test
  public void definition_callee_symbol() {
    FileInput tree = parse(
      "def fn(): pass",
      "fn('foo')"
    );
    assertNameAndQualifiedName(tree, "fn", null);
  }