RenderField in traitView does not handle null or undefined

Hi @artf!

Working on the editor on one of my custom traits i had an issue where for some reason that is not important, the createInput was returning undefined.

I've noticed that the TraitView renderField method does not handle falsey values

renderField() {
   const { $el, appendInput, model } = this;
   const inputs = $el.find('[data-input]');
   const el = inputs[inputs.length - 1];
   let tpl = model.el;

   if (!tpl) {
     tpl = this.createInput
       ? this.createInput(this.getClbOpts())
       : this.getInputEl();
   }

   if (isString(tpl)) {
     el.innerHTML = tpl;
     this.elInput = el.firstChild;
   } else {
     appendInput ? el.appendChild(tpl) : el.insertBefore(tpl, el.firstChild);
     this.elInput = tpl;
   }

   model.el = this.elInput;
 }

It when directly to try to append an undefined object, raising an error.

Should i add a validation here checking that the object is present or should i do it somewhere else? Should it be the responsibility of createInput to always return a defined object?

Thanks!