In version 1.3.20.0 when I did assigning a template to 'NuPattern.Library.Commands.GenerateProductCodeCommand' then I did just see this error:
NuPattern.Library.Design.TextTemplateUriEditor Error: 0 : Failed to edit file. System.ArgumentException: The solution element 'PatternToolkit3' must be contained in a project containing a VSIX manifest file marked with the 'IsToolkitManifest' MSBuild metadata. at NuPattern.VisualStudio.Solution.SolutionExtensions.GetToolkitManifest(IProject project) at NuPattern.VisualStudio.Solution.SolutionExtensions.GetToolkitManifest(IItemContainer element) at NuPattern.Library.Design.TextTemplateUriEditor.BuildUri(IItemContainer selectedItem) at NuPattern.Library.Design.TextTemplateUriEditor.<>c_DisplayClass4.b_3() at NuPattern.VisualStudio.TraceSourceExtensions.DoShield(ITracer traceSource, Action action, String format, Boolean showUI, String[] args)