Form designer cannot be loaded

Question

I have a project that I am working about a year old. It has been running fine but needs a little love. One of these is to use only x64 on some of the needed libraries.

Now that I have been working on this a bit I cannot load two forms that derive from a base class. This worked before, I am not sure what I did to break this.

My error is:

The designer could not be shown for this file because none of the classes within it can be designed. The designer inspected the following classes in the file: Form1 --- The base class 'ThermalVision.ExtendedForm' could not be loaded. Ensure the assembly has been referenced and that all projects have been built.

My extended for form loads and the design loads just fine. The Extended form is super simple:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace ThermalVision
{
    public class ExtendedForm : Form
    {
        public delegate void BroadcastMouseLocation(object sender, EventArgs e);
        public event BroadcastMouseLocation OnBroadcastMouseLocation;


        public void ReceivedMouseHover(object sender, EventArgs e)
        {
            MousePointEventArgument mpea = (MousePointEventArgument)e;
            OnBroadcastMouseLocation(this, mpea);
        }
    }
}

I have tried deleting the Extended for from the project and readding. I do see other solutions out there but they seem not related to this issue.