toolbar/RunButton: enable only when active file

Before multi-file support, it was not possible to have no open file.
So we need to change the state logic to only enable the button when
there is an active file rather than when the editor "is ready".

Fixes: https://github.com/pybricks/support/issues/691

src/toolbar/buttons/run/RunButton.test.tsx

@@ -3,7 +3,7 @@
 
 import { cleanup } from '@testing-library/react';
 import React from 'react';
-import { testRender } from '../../../../test';
+import { testRender, uuid } from '../../../../test';
 import { downloadAndRun } from '../../../hub/actions';
 import { HubRuntimeState } from '../../../hub/reducers';
 import RunButton from './RunButton';
@@ -14,7 +14,7 @@ afterEach(() => {
 
 it('should dispatch action when clicked', async () => {
     const [user, button, dispatch] = testRender(<RunButton id="test-run-button" />, {
-        editor: { isReady: true },
+        editor: { activeFileUuid: uuid(0) },
         hub: { runtime: HubRuntimeState.Idle },
     });
 

src/toolbar/buttons/run/RunButton.tsx

@@ -16,7 +16,7 @@ const RunButton: React.VoidFunctionComponent<RunButtonProps> = ({ id }) => {
     const downloadProgress = useSelector((s) => s.hub.downloadProgress);
     const mpyAbiVersion = useSelector((s) => s.hub.mpyAbiVersion);
     const runtime = useSelector((s) => s.hub.runtime);
-    const isEditorReady = useSelector((s) => s.editor.isReady);
+    const activeFile = useSelector((s) => s.editor.activeFileUuid);
     const keyboardShortcut = 'F5';
 
     const i18n = useI18n();
@@ -35,7 +35,7 @@ const RunButton: React.VoidFunctionComponent<RunButtonProps> = ({ id }) => {
                     : i18n.translate('tooltip.action', { key: keyboardShortcut })
             }
             icon={icon}
-            enabled={isEditorReady && runtime === HubRuntimeState.Idle}
+            enabled={activeFile !== null && runtime === HubRuntimeState.Idle}
             showProgress={runtime === HubRuntimeState.Loading}
             progress={downloadProgress === null ? undefined : downloadProgress}
             onAction={() => dispatch(downloadAndRun(mpyAbiVersion))}