Move pagination code to pagination.cs

Add error handler if _MyStepRTB is null

PROMS/Volian.Controls.Library/StepTabRibbon.cs

@@ -177,6 +177,7 @@ namespace Volian.Controls.Library
 
 		void _MyStepRTB_TextChanged(object sender, EventArgs e)
 		{
+			if (_MyStepRTB == null) return;
 			// if we are in view mode or the edit window is empty, then don't allow inserting steps/substeps,
 			// page breaks, copy step, nor creating a pdf
 			bool allow = (_MyStepRTB.TextLength > 0);