Handle null case in ReferencesCodeLensProvider's CodeLens resolver

This change fixes an issue where a null case isn't being handled
gracefully in the ReferencesCodeLensProvider when resolving a CodeLens
while the user is typing out a new function defintion or doing anything
else that causes the code to change like formatting the script.

Resolves PowerShell/vscode-powershell#857
Resolves PowerShell/vscode-powershell#855

Author: daviwil

src/PowerShellEditorServices.Host/CodeLens/CodeLensFeature.cs

@@ -147,7 +147,8 @@ await requestContext.SendResult(
                 }
                 else
                 {
-                    // TODO: Write error!
+                    await requestContext.SendError(
+                        $"Could not find provider for the original CodeLens: {codeLensData.ProviderId}");
                 }
             }
         }

src/PowerShellEditorServices.Host/CodeLens/ReferencesCodeLensProvider.cs

@@ -64,16 +64,18 @@ await editorSession.LanguageService.FindReferencesOfSymbol(
                     references,
                     editorSession.Workspace);
 
-            var referenceLocations =
-                referencesResult
-                    .FoundReferences
-                    .Select(
-                        r => new Location
-                        {
-                            Uri = GetFileUri(r.FilePath),
-                            Range = r.ScriptRegion.ToRange()
-                        })
-                    .ToArray();
+            Location[] referenceLocations =
+                referencesResult == null
+                    ? new Location[0]
+                    : referencesResult
+                        .FoundReferences
+                        .Select(
+                            r => new Location
+                            {
+                                Uri = GetFileUri(r.FilePath),
+                                Range = r.ScriptRegion.ToRange()
+                            })
+                        .ToArray();
 
             return
                 new CodeLens(