Fix crash on joining rooms

#6818 passed roomType
in obbData & then used oobData without null checking, but we don't always
have oobData so joining a room from /join or from an invite would crash.

Type: defect

src/components/views/rooms/RoomPreviewBar.tsx

@@ -323,7 +323,7 @@ export default class RoomPreviewBar extends React.Component<IProps, IState> {
         const messageCase = this.getMessageCase();
         switch (messageCase) {
             case MessageCase.Joining: {
-                title = this.props.oobData.roomType === RoomType.Space ? _t("Joining space …") : _t("Joining room …");
+                title = this.props.oobData?.roomType === RoomType.Space ? _t("Joining space …") : _t("Joining room …");
                 showSpinner = true;
                 break;
             }