Most likely you uninstalled the product before you clean up the form which uses it. This will confuse the designer as it is not able to find all types referenced by the form. To fix this you need to manually clean up all designer generated code and resources related to the control found in Form.Designer.cs (or the like). Typically the code you need to delete looks like:
this.nChartControl1 = new Nevron.Chart.WinForm.NChartControl();
this.nChartControl1.AutoRefresh = false;
this.nChartControl1.BackColor = System.Drawing.SystemColors.Control;
this.nChartControl1.InputKeys = new System.Windows.Forms.Keys;
this.nChartControl1.Location = new System.Drawing.Point(12, 5);
this.nChartControl1.Name = "nChartControl1";
this.nChartControl1.Size = new System.Drawing.Size(461, 318);
this.nChartControl1.State = ((Nevron.Chart.WinForm.NState)(resources.GetObject("nChartControl1.State")));
this.nChartControl1.TabIndex = 1;
this.nChartControl1.Text = "nChartControl1";
private Nevron.Chart.WinForm.NChartControl nChartControl1;
In addition you can view the resource file generated by the designer (like Form1.resx) and remove all appearances of nChartControl(N).State there.
As a last resort you can simply recreate the form.
Nevron Support Team