[BUG] Getting a 404 on mixins.js

First of all, thank you for this great tool.

1. I'm using the latest version.
2. The bug happens with a local copy of GrapesJS.
   1. MacOS - latest Chrome browser
   2. The bug is not present on the demo
3. Expected behavior: have the editor loading without any errors
4. What happens instead: I get a 404 error when the editor is appending styles

I'm getting the following error on the console after the editor is started: mixins.js:27 GET http://example.com/editor/null net::ERR_ABORTED 404 (Not Found) I replaced the real domain with "example.com", the rest of the URL is real.

On mixins.js, line 27, the instruction is head.insertBefore(link, head.firstChild);, it's part of the function that append styles to the editor. The problem is I'm passing an array of valid URLs, all of them resolve correctly as I see each style applied to the editor's html.

Here's how I'm creating the editor:

var editorSettings = {
	fromElement: true,
	height: '900px',
	width: '600px',
	cssIcons: null,
	storageManager: { type: null },
	noticeOnUnload: false,
	panels: { defaults: [] },
	canvas: {
		styles: [
			'https://fonts.googleapis.com/css?family=Libre+Franklin:400,600,700&display=swap',
			'https://stackpath.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css',
			'/wp-content/plugins/votersguide/public/css/pageTemplate.css?version=1.0.26'
		]
	},
	assetManager: {
		storageType  	: '',
		storeOnChange  : true,
		storeAfterUpload  : true,
		assets    	: [],
		uploadFile: function(e) {
			var postHash = getUrlVars()['g'];
			var nonce = $('#wpNonce').val();
			var files = e.dataTransfer ? e.dataTransfer.files : e.target.files;
			var formData = new FormData();
			for(var i in files){
				formData.append('file-'+i, files[i]) //containing all the selected images from local
			}
			formData.append('action', 'votersguide_editor');
			formData.append('g', postHash);
			formData.append('nonce', nonce);

			$.ajax({
				url: myAjax.ajaxurl,
				type: 'POST',
				data: formData,
				contentType: false,
				crossDomain: true,
				dataType: 'json',
				mimeType: "multipart/form-data",
				processData:false,
				success: function(result){
					var myJSON = [];
					$.each( result['data'], function( key, value ) {
						myJSON[key] = value;
					});
					var images = myJSON;
					editor.AssetManager.add(images); //adding images to asset manager of GrapesJS
				}
			});
		},
	},
};

editorSettings.container = '#'+selectedPage.attr('id');

var editor = grapesjs.init(editorSettings);