Error while documentation editing (#231)

Hello.

I'm trying to place caret in the end of documentation and I get an ArgumentOutOfRangeException.

Here is the call stack:

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.

Parameter name: startIndex

at System.String.Insert(Int32 startIndex, String value)

at SoftwareIdeasModeler.Core.TextRendering.FormattedText.InsertText(String text, Int32 index)

at SoftwareIdeasModeler.Controls.FormattedTextBox.Commands.InsertTextCommand.Execute()

at SoftwareIdeasModeler.Core.Projecting.CommandStack.AddCommand(Command command, Boolean execute)

at SoftwareIdeasModeler.Core.Projecting.CommandStack.Execute(Command command)

at SoftwareIdeasModeler.Controls.FormattedTextBox.FormattedTextBox.InsertText(String text)

at SoftwareIdeasModeler.Controls.FormattedTextBox.FormattedTextBox.InsertCharacter(Char ch)

at SoftwareIdeasModeler.Controls.FormattedTextBox.FormattedTextBox.ProcessKeyPress(Char c)

at SoftwareIdeasModeler.Controls.FormattedTextBox.FormattedTextBox.ProcessMnemonic(Char charCode)

at System.Windows.Forms.ContainerControl.ProcessDialogChar(Char charCode)

at System.Windows.Forms.Control.ProcessDialogChar(Char charCode)

at System.Windows.Forms.ContainerControl.ProcessDialogChar(Char charCode)

at System.Windows.Forms.Control.PreProcessMessage(Message& msg)

at System.Windows.Forms.Control.PreProcessControlMessageInternal(Control target, Message& msg)

at System.Windows.Forms.Application.ThreadContext.PreTranslateMessage(MSG& msg)