add one more test case, adjust err msg

crates/nu-command/src/generators/generate.rs

@@ -205,7 +205,7 @@ fn get_initial_state(
                     msg: "Missing intial value".to_string(),
                     span: Some(span),
                     help: Some(
-                        "Provide initial value in generate, or assigning default value to closure parameter"
+                        "Providing <initial> argument to generate, or assigning default value to closure parameter"
                             .to_string(),
                     ),
                     inner: vec![],

crates/nu-command/tests/commands/generate.rs

@@ -132,3 +132,19 @@ fn generate_allow_default_parameter() {
     ));
     assert_eq!(actual.out, "[null, null, done]");
 }
+
+#[test]
+fn generate_raise_error_on_no_default_parameter_closure_and_init_val() {
+    let actual = nu!(pipeline(
+        r#"
+        generate {|x|
+          if $x == 3 {
+            {out: "done"}
+          } else {
+            {out: null, next: ($x + 1)}
+          }
+        } | to nuon
+      "#
+    ));
+    assert_eq!(actual.err, "The initial value is missing");
+}