list_render: Error if opts are missing.

The check here was too late, and it should
have given a blueslip error.  We obviously
don't expect these errors at runtime; this
is a convenience for developers creating
new widgets.

frontend_tests/node_tests/list_render.js

@@ -453,3 +453,13 @@ run_test('clear_event_handlers', () => {
     assert.equal(scroll_container.cleared, true);
     assert.equal(filter_element.cleared, true);
 });
+
+run_test('errors', () => {
+    // We don't care about actual data for this test.
+    const list = 'stub';
+    const container = 'stub';
+
+    blueslip.expect('error', 'Need opts to create widget.');
+    list_render.create(container, list);
+    blueslip.reset();
+});

static/js/list_render.js

@@ -51,6 +51,11 @@ exports.validate_filter = (opts) => {
 // list: The list of items to progressively append.
 // opts: An object of random preferences.
 exports.create = function ($container, list, opts) {
+    if (!opts) {
+        blueslip.error('Need opts to create widget.');
+        return;
+    }
+
     if (opts.name && DEFAULTS.instances.get(opts.name)) {
         // Clear event handlers for prior widget.
         const old_widget = DEFAULTS.instances.get(opts.name);
@@ -68,9 +73,6 @@ exports.create = function ($container, list, opts) {
         filter_value: '',
     };
 
-    if (!opts) {
-        return;
-    }
     exports.validate_filter(opts);
 
     const widget = {};