Allow null parser state to facilitate easier testing.

Author: comintern

Rubberduck.Core/Navigation/CodeExplorer/CodeExplorerReferenceFolderViewModel.cs

@@ -72,6 +72,7 @@ public void Synchronize(Declaration parent, List<ReferenceModel> updated)
             foreach (var reference in updates)
             {
                 reference.IsUsed = reference.IsBuiltIn ||
+                                   _finder != null &&
                                    _finder.IsReferenceUsedInProject(Declaration as ProjectDeclaration,
                                        reference.ToReferenceInfo());
 
@@ -95,6 +96,7 @@ public void UpdateChildren()
                 }
 
                 reference.IsUsed = reference.IsBuiltIn ||
+                                   _finder != null &&
                                    _finder.IsReferenceUsedInProject(
                                        library.Parent?.Declaration as ProjectDeclaration,
                                        reference.ToReferenceInfo());