Exception while generating documentation (#407)

I tried to use the "generate interactive documentation" feature for the first time. This caused the following exception:

System.NullReferenceException: Object reference not set to an instance of an object.

at SoftwareIdeasModeler.Core.TextRendering.TextRenderer.Calculate()

at SoftwareIdeasModeler.Core.Generation.TemplateEvaluating.TemplateEvaluator.GetHtmlDocumentation(FormattedText documentation)

at SoftwareIdeasModeler.Core.Generation.TemplateEvaluating.TemplateEvaluator.EvaluateConcreteItemCommand(ConcreteDiagramItem item, CommandWithParameters command)

at SoftwareIdeasModeler.Core.Generation.TemplateEvaluating.TemplateEvaluator.EvaluateCommand(CommandWithParameters command, Boolean isForEach)

at SoftwareIdeasModeler.Core.Generation.TemplateEvaluating.TemplateEvaluator.EvaluateStatement(String statement)

at SoftwareIdeasModeler.Core.Generation.TemplateEvaluating.TemplateEvaluator.Evaluate()

at SoftwareIdeasModeler.Core.Generation.Templates.TemplateFileProcessor.Process(Project project)

at SoftwareIdeasModeler.Core.Generation.Templates.TemplateProcessor.ProcessDiagram(TemplateFileItem templateFile, TemplateFileProcessor ftProcessor, Diagram diagram)

at SoftwareIdeasModeler.Core.Generation.Templates.TemplateProcessor.ProcessTemplateFileItem(String templateDirectory, TemplateFileItem templateFileItem, Object obj)

at SoftwareIdeasModeler.Core.Generation.Templates.TemplateProcessor.ProcessDiagram(Diagram diagram)

at SoftwareIdeasModeler.Dialogs.Generating.InteractiveDocumentation.InteractiveDocumentationWriter.ProcessModel(TemplateProcessor processor, Model model)

at SoftwareIdeasModeler.Dialogs.Generating.InteractiveDocumentation.InteractiveDocumentationWriter.Save(String filename)

at SoftwareIdeasModeler.Dialogs.Generating.InteractiveDocumentation.DocumentationForm.generateButton_Click(Object sender, EventArgs e)

at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)

at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)

at System.Windows.Forms.Control.WndProc(Message& m)

at System.Windows.Forms.ButtonBase.WndProc(Message& m)

at System.Windows.Forms.Button.WndProc(Message& m)

at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)