Fix null reference error in evaluator

Upsilon/Evaluator/Evaluator.cs

@@ -632,7 +632,11 @@ namespace Upsilon.Evaluator
                 while (enumerator.MoveNext())
                 {
                     var current = enumerator.Current;
-                    if (current == null || current.Type != Type.Table)
+                    if (current == null)
+                    {
+                        throw new Exception($"Can't assign result value of nothing to multiple values");
+                    }
+                    if (current.Type != Type.Table)
                     {
                         throw new Exception($"Can't assign result value with type '{current.Type}' to multiple values");
                     }