Why do I get error msg " Method does not exist or incorrect signature: void getRef() from the type String at line 6 column 30" for test class?

the goal of my test class is simply to compare one url to another url, please see code below:

@isTest private class test {
    static void filePreviewUrl_test() {

    String testId = 'thistestid';
    String url = fxl_controller_reactGrid.filePreviewUrl(testId);
    String testUrl = URL.getSalesforceBaseUrl().toExternalForm()+'/lightning/r/ContentDocument/thistestid/view';


    System.assertEquals(url, testUrl);
}

}