Check for server support on state future send

Needed because this check is otherwise lost by the override

Author: AndrewFerr

src/client.ts

@@ -530,7 +530,7 @@ export const UNSTABLE_MSC2666_SHARED_ROOMS = "uk.half-shot.msc2666";
 export const UNSTABLE_MSC2666_MUTUAL_ROOMS = "uk.half-shot.msc2666.mutual_rooms";
 export const UNSTABLE_MSC2666_QUERY_MUTUAL_ROOMS = "uk.half-shot.msc2666.query_mutual_rooms";
 
-const UNSTABLE_MSC4140_FUTURES = "org.matrix.msc4140";
+export const UNSTABLE_MSC4140_FUTURES = "org.matrix.msc4140";
 
 enum CrossSigningKeyType {
     MasterKey = "master_key",

src/embedded.ts

@@ -42,6 +42,7 @@ import {
     IStartClientOpts,
     SendToDeviceContentMap,
     IOpenIDToken,
+    UNSTABLE_MSC4140_FUTURES,
 } from "./client";
 import { SyncApi, SyncState } from "./sync";
 import { SlidingSyncSdk } from "./sliding-sync-sdk";
@@ -330,6 +331,10 @@ export class RoomWidgetClient extends MatrixClient {
         content: StateEvents[K],
         stateKey = "",
     ): Promise<SendFutureResponse<F>> {
+        if (!(await this.doesServerSupportUnstableFeature(UNSTABLE_MSC4140_FUTURES))) {
+            throw Error("Server does not support the Futures API");
+        }
+
         // TODO: better type checking
         return (await this.widgetApi.sendStateEvent(
             eventType,