types: Migrated add_stream_options_popover to TypeScript.

The migration requires some new assert statements as well as a change
in how the `return undefined` case is spelled to match the types declared by 
Tippy upstream.

tools/test-js-with-node

@@ -46,7 +46,7 @@ EXEMPT_FILES = make_set(
     [
         "web/shared/src/poll_data.ts",
         "web/src/about_zulip.ts",
-        "web/src/add_stream_options_popover.js",
+        "web/src/add_stream_options_popover.ts",
         "web/src/add_subscribers_pill.js",
         "web/src/admin.js",
         "web/src/alert_popup.ts",

web/src/add_stream_options_popover.ts

@@ -1,4 +1,6 @@
 import $ from "jquery";
+import assert from "minimalistic-assert";
+import type {ReferenceElement} from "tippy.js";
 
 import render_left_sidebar_stream_setting_popover from "../templates/popovers/left_sidebar_stream_setting_popover.hbs";
 
@@ -6,7 +8,7 @@ import * as popover_menus from "./popover_menus";
 import * as settings_data from "./settings_data";
 import {parse_html} from "./ui_util";
 
-export function initialize() {
+export function initialize(): void {
     popover_menus.register_popover_menu("#streams_inline_icon", {
         onShow(instance) {
             const can_create_streams =
@@ -31,32 +33,41 @@ export function initialize() {
             //  When showing the popover menu, we want the
             // "Add streams" and the "Filter streams" tooltip
             //  to appear below the "Add streams" icon.
-            const add_streams_tooltip = $("#add_streams_tooltip").get(0);
+            const add_streams_tooltip: ReferenceElement | undefined =
+                $("#add_streams_tooltip").get(0);
+            assert(add_streams_tooltip !== undefined);
             add_streams_tooltip._tippy?.setProps({
                 placement: "bottom",
             });
-            const filter_streams_tooltip = $("#filter_streams_tooltip").get(0);
+
+            const filter_streams_tooltip: (ReferenceElement & HTMLElement) | undefined =
+                $("#filter_streams_tooltip").get(0);
             // If `filter_streams_tooltip` is not triggered yet, this will set its initial placement.
+            assert(filter_streams_tooltip !== undefined);
             filter_streams_tooltip.dataset.tippyPlacement = "bottom";
             filter_streams_tooltip._tippy?.setProps({
                 placement: "bottom",
             });
 
-            // The linter complains about unbalanced returns
+            return undefined;
-            return true;
         },
         onHidden(instance) {
             instance.destroy();
-            popover_menus.popover_instances.stream_settings = undefined;
+            popover_menus.popover_instances.stream_settings = null;
             //  After the popover menu is closed, we want the
             //  "Add streams" and the "Filter streams" tooltip
             //  to appear at it's original position that is
             //  above the "Add streams" icon.
-            const add_streams_tooltip = $("#add_streams_tooltip").get(0);
+            const add_streams_tooltip: ReferenceElement | undefined =
+                $("#add_streams_tooltip").get(0);
+            assert(add_streams_tooltip !== undefined);
             add_streams_tooltip._tippy?.setProps({
                 placement: "top",
             });
-            const filter_streams_tooltip = $("#filter_streams_tooltip").get(0);
+
+            const filter_streams_tooltip: (ReferenceElement & HTMLElement) | undefined =
+                $("#filter_streams_tooltip").get(0);
+            assert(filter_streams_tooltip !== undefined);
             filter_streams_tooltip.dataset.tippyPlacement = "top";
             filter_streams_tooltip._tippy?.setProps({
                 placement: "top",