Steps to reproduce:

• As an example try this page: https://de.wikipedia.org/w/index.php?title=Scalable_Vector_Graphics&veaction=edit&vesection=2
• It has a code block for an example of an svg image. Click on it
• Click "Edit"
• Change anything (e.g. tick the "Show line numbers" checkbox
• Click "Done"
• Now, click edit again (you can, if you want, choose another code block, too, or try to add a new code block)

Expected result:
The code editor should be displayed again

Observed result:
Nothing happens

As a nice gimmick, you aren't able to get the save dialog, or the "Switch to source editor", so any of your made edits is lost now :) (Maybe you can cross copy it to another tab where you started VE, but that isn't a good solution :P).

In the browser console you get:
"Uncaught TypeError: Cannot read property 'fail' of null" in ve.ui.MWAceEditorWidget.js:220, which is:

ve.ui.MWAceEditorWidget.prototype.adjustSize = function ( force ) {
	var widget = this;
	// If the editor has loaded, resize events are emitted from #onEditorResize
	// so do nothing here unless this is a user triggered resize, otherwise call the parent method.
	if ( force ) {
		this.loadingPromise.done( function () {
			widget.editor.resize();
		} );
	}
	this.loadingPromise.fail( function () { // <---- Line 220
		// Parent method
		ve.ui.MWAceEditorWidget.super.prototype.adjustSize.call( widget );
	} );
};