Modal: remove support for onFinished callback (#29852)

* Fix up type for `finished` result of Modal

The `finished` promise can be called with an empty array, for example if the
dialog is closed by a background click. This was not correctly represented in
the typing. Fix that, and add some documentation while we're at it.

* Type fixes to onFinished callbacks from Modal

These can all be called with zero arguments, despite what the type annotations
may say, so mark them accordingly.

* Remove uses of Modal `onFinished` property

... because it is confusing.

Instead, use the `finished` promise returned by `createDialog`.

* Modal: remove support for now-unused `onFinished` prop

* StopGapWidgetDriver: use `await` instead of promise chaining

* Fix up unit tests

src/utils/DialogOpener.ts

@@ -119,7 +119,7 @@ export class DialogOpener {
                 break;
             case Action.OpenAddToExistingSpaceDialog: {
                 const space = payload.space;
-                Modal.createDialog(
+                const { finished } = Modal.createDialog(
                     AddExistingToSpaceDialog,
                     {
                         onCreateRoomClick: (ev: ButtonEvent) => {
@@ -128,14 +128,14 @@ export class DialogOpener {
                         },
                         onAddSubspaceClick: () => showAddExistingSubspace(space),
                         space,
-                        onFinished: (added: boolean) => {
-                            if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
-                                defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
-                            }
-                        },
                     },
                     "mx_AddExistingToSpaceDialog_wrapper",
                 );
+                finished.then(([added]) => {
+                    if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
+                        defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
+                    }
+                });
                 break;
             }
         }

src/utils/leave-behaviour.ts

@@ -171,20 +171,20 @@ export async function leaveRoomBehaviour(
 }
 
 export const leaveSpace = (space: Room): void => {
-    Modal.createDialog(
+    const { finished } = Modal.createDialog(
         LeaveSpaceDialog,
         {
             space,
-            onFinished: async (leave: boolean, rooms: Room[]): Promise<void> => {
-                if (!leave) return;
-                await bulkSpaceBehaviour(space, rooms, (room) => leaveRoomBehaviour(space.client, room.roomId));
-
-                dis.dispatch<AfterLeaveRoomPayload>({
-                    action: Action.AfterLeaveRoom,
-                    room_id: space.roomId,
-                });
-            },
         },
         "mx_LeaveSpaceDialog_wrapper",
     );
+    finished.then(async ([leave, rooms]) => {
+        if (!leave) return;
+        await bulkSpaceBehaviour(space, rooms!, (room) => leaveRoomBehaviour(space.client, room.roomId));
+
+        dis.dispatch<AfterLeaveRoomPayload>({
+            action: Action.AfterLeaveRoom,
+            room_id: space.roomId,
+        });
+    });
 };

src/utils/space.tsx

@@ -75,7 +75,7 @@ export const showCreateNewRoom = async (space: Room, type?: RoomType): Promise<b
     });
     const [shouldCreate, opts] = await modal.finished;
     if (shouldCreate) {
-        await createRoom(space.client, opts);
+        await createRoom(space.client, opts!);
     }
     return !!shouldCreate;
 };
@@ -106,35 +106,35 @@ export const showSpaceInvite = (space: Room, initialText = ""): void => {
 };
 
 export const showAddExistingSubspace = (space: Room): void => {
-    Modal.createDialog(
+    const { finished } = Modal.createDialog(
         AddExistingSubspaceDialog,
         {
             space,
             onCreateSubspaceClick: () => showCreateNewSubspace(space),
-            onFinished: (added: boolean) => {
-                if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
-                    defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
-                }
-            },
         },
         "mx_AddExistingToSpaceDialog_wrapper",
     );
+    finished.then(([added]) => {
+        if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
+            defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
+        }
+    });
 };
 
 export const showCreateNewSubspace = (space: Room): void => {
-    Modal.createDialog(
+    const { finished } = Modal.createDialog(
         CreateSubspaceDialog,
         {
             space,
             onAddExistingSpaceClick: () => showAddExistingSubspace(space),
-            onFinished: (added: boolean) => {
-                if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
-                    defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
-                }
-            },
         },
         "mx_CreateSubspaceDialog_wrapper",
     );
+    finished.then(([added]) => {
+        if (added && SdkContextClass.instance.roomViewStore.getRoomId() === space.roomId) {
+            defaultDispatcher.fire(Action.UpdateSpaceHierarchy);
+        }
+    });
 };
 
 export const bulkSpaceBehaviour = async (