Size is a string, import react

React gets put in by the JSX transpile

src/components/views/elements/StartChatButton.js

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import React from 'react';
 import sdk from '../../../index';
 import PropTypes from 'prop-types';