Ensure the control namespace and assembly are correct. Use the exception message. For example:
"Type "SitefinityWebApp.Widgets.MyWidget, SitefinityWebApp" cannot be resolved."
Verify that there is a control MyWidget and its namespace is SitefinityWebApp.Widgets and it is in the SitefinityWebApp project.
Ensure the control is included in the project and if not, then include it by right-clicking the file and selecting "Include in project".
Build the project to compile the control in the assembly.
If the control type name or namespace has been changed, set the correct ones in the Toolbox: Go to Advanced settings -> Toolboxes -> Toolboxes -> PageControls -> Sections, find the section of the control -> Tools, select the control and edit its Type.