Deflake verification playwright tests (#12669)

Turns out the sleep that got removed in
https://github.com/matrix-org/matrix-react-sdk/pull/12667 was useful.

playwright/e2e/crypto/utils.ts

@@ -113,6 +113,13 @@ export async function checkDeviceIsConnectedKeyBackup(
     expectedBackupVersion: string,
     checkBackupKeyInCache: boolean,
 ): Promise<void> {
+    // Sanity check the given backup version: if it's null, something went wrong earlier in the test.
+    if (!expectedBackupVersion) {
+        throw new Error(
+            `Invalid backup version passed to \`checkDeviceIsConnectedKeyBackup\`: ${expectedBackupVersion}`,
+        );
+    }
+
     await page.getByRole("button", { name: "User menu" }).click();
     await page.locator(".mx_UserMenu_contextMenu").getByRole("menuitem", { name: "Security & Privacy" }).click();
     await expect(page.locator(".mx_Dialog").getByRole("button", { name: "Restore from Backup" })).toBeVisible();

playwright/e2e/crypto/verification.spec.ts

@@ -50,11 +50,17 @@ test.describe("Device verification", () => {
             bootstrapSecretStorage: true,
         });
         aliceBotClient.setCredentials(credentials);
-        const mxClientHandle = await aliceBotClient.prepareClient();
 
-        expectedBackupVersion = await mxClientHandle.evaluate(async (mxClient) => {
-            return await mxClient.getCrypto()!.getActiveSessionBackupVersion();
-        });
+        // Backup is prepared in the background. Poll until it is ready.
+        const botClientHandle = await aliceBotClient.prepareClient();
+        await expect
+            .poll(async () => {
+                expectedBackupVersion = await botClientHandle.evaluate((cli) =>
+                    cli.getCrypto()!.getActiveSessionBackupVersion(),
+                );
+                return expectedBackupVersion;
+            })
+            .not.toBe(null);
     });
 
     // Click the "Verify with another device" button, and have the bot client auto-accept it.