[Debugger] Handle NRE when adding a watch when not debugging

Fixes VSTS 1025766
https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1025766

Author: sgmunn

main/src/addins/MonoDevelop.Debugger/MonoDevelop.Debugger/ObjectValue/ObjectValueTreeViewController.cs

@@ -413,7 +413,15 @@ public void AddExpression (string expression)
 			}
 
 			LoggingService.LogInfo ("Evaluating expression '{0}'", expression);
-			var node = Frame.EvaluateExpression (expression);
+
+			ObjectValueNode node;
+			if (Frame != null) {
+				node = Frame.EvaluateExpression (expression);
+			} else {
+				var value = ObjectValue.CreateUnknown (expression);
+				node = new DebuggerObjectValueNode (value);
+			}
+
 			AddValue (node);
 
 			ExpressionAdded?.Invoke (this, new ExpressionAddedEventArgs (expression));