B2021-122: Crash on expand of search panel/select of RNO Only with nothing in editor

PROMS/Volian.Controls.Library/DisplaySearch.cs

@@ -2183,6 +2183,9 @@ namespace Volian.Controls.Library
 			// B2021 - 100: Enable / disable the RNO Only checkbox &WORD Sections tree node in Step Elements to Search.
 			if (tabSearchTypes.SelectedTab == tabTranSearch) return;
 
+			// B2021-122: null reference crash if clicking RNOOnly before anything else, i.e. tree set up with types.
+			if (wordSectionTreeNode == null) return;
+
 			wordSectionTreeNode.Enabled = true;
 			if (!cbxRnoOnly.Checked)    // checked turning on
 			{