check for null reference when deleting a source step that has associated enhanced (found during fix for B2016-228)

B2016-228: when creating section/procedure enhanced items, set ‘Number’
B2016-228: when modifying source section or procedure, save ‘Number’ also
B2016-198: User hold point no longer overwrites the ‘INIT/DATE’ signoff heading

PROMS/Volian.Print.Library/vlnParagraph.cs

@@ -2558,6 +2558,7 @@ namespace Volian.Print.Library
 			get
 			{
 				if (TopMostChild.PartsAbove.Count == 0) return false;
+				if (TopMostChild.PartsContainer != null && TopMostChild.PartsContainer.Count > 0) return false;	// B2016-198: not 'EmptyTopMostPart' if there is a box.
 				foreach (vlnPrintObject po in TopMostChild.PartsAbove)
 				{
 					vlnHeader hd = po as vlnHeader;