How to load the framework programmatically without errors?

Hi,

for some reason, it would be helpfull, to laod all necessary scripts and css files programatically.

So I tried this code (which is in one file, which is referenced in the header of the document):

function addJsFile(_file){
    let _JsFile = document.createElement('script');
    _JsFile.src = _file;
    document.head.appendChild(_JsFile);
}

function addCssFile(_file){
    let _CssFile = document.createElement('link');
    _CssFile.href = _file;
    _CssFile.type = "text/css";
    _CssFile.rel = "stylesheet";
    document.head.appendChild(_CssFile );
}

function load_mdb_framework(){
    addJsFile("./approot/jquery/mdb_pro/js/jquery-3.3.1.min.js");
    addJsFile("./approot/jquery/mdb_pro/js/popper.min.js"); 
    addJsFile("./approot/jquery/mdb_pro/js/bootstrap.min.js");  
    addJsFile("./approot/jquery/mdb_pro/js/mdb.min.js");
    addCssFile("./approot/jquery/mdb_pro/css/bootstrap.min.css");
    addCssFile("./approot/jquery/mdb_pro/css/mdb.min.css");
}
load_mdb_framework();

If I run the page, I have some error messages in the console (IE and Chrrome) like: Unable to get property 'addEventListener' of undefined or null reference (mdb.min.js, Line: 1, Column: 204609)

Can you help with this or explain, why it doesn't work? Thanks