Hi, sorry to hear that.
We cannot reproduce the issue on AlloyMVC template. Could you please create a SupportCase? We need your environment to dig deeper in this issue. Please give us full information about your CMS version, your other saterlites modules/addon, ... and your project if possible.
I just installed EPiServer Forms 3.1.0 and I got this error, which according to buglist should be solved. Occurs every time I try to drag and drop a new basic element to the form elemnet container. Also, I need to save the form every time I add a new formelement from the container or it might be removed by itself for some reason.