Throw error if for some reason the execution of a statement in a coroutine returns null

Upsilon/Binder/BoundStatements/BoundBlockStatement.cs

@@ -39,6 +39,11 @@ namespace Upsilon.Binder
             foreach (var statement in Statements)
             {
                 var coroutine = statement.EvaluateCoroutine(scope, diagnostics, state);
+                if (coroutine == null)
+                {
+                    diagnostics.LogError("Can't evaluate a statement, it returned nil.", statement.Span);
+                    continue;
+                }
                 while (coroutine.MoveNext())
                 {
                     yield return coroutine.Current;