On adding a visual web part I get this error "Key cannot be null. Parameter name: key"

Question

I've created a SharePoint 2010 Visual Web Part project.

Added some html to the ascx file and deployed it.

When I try to add it to my page I get this error - Key cannot be null. Parameter name: key

<%@ Assembly Name="$SharePoint.Project.AssemblyFullName$" %>
<%@ Assembly Name="Microsoft.Web.CommandUI, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %> 
<%@ Register Tagprefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %> 
<%@ Register Tagprefix="Utilities" Namespace="Microsoft.SharePoint.Utilities" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register Tagprefix="asp" Namespace="System.Web.UI" Assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" %>
<%@ Import Namespace="Microsoft.SharePoint" %> 
<%@ Register Tagprefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPages" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="VisualWebPart1UserControl.ascx.cs" Inherits="VisualWebPartProject1.VisualWebPart1.VisualWebPart1UserControl" %>

<%@ Import Namespace="Microsoft.SharePoint.WebControls" %> 

<div>This is my .... </div>

<div> ended.... </div>

Answer 1

Have seen this sometimes when moving project to another computer och while renaming stuff in the project. Try and recreate from scratch and see if it still exists.