Size is a string, import react

React gets put in by the JSX transpile

src/components/views/elements/ActionButton.js

@@ -33,7 +33,7 @@ export default React.createClass({
 
     getDefaultProps: function() {
         return {
-            size: 25,
+            size: "25",
             tooltip: false,
         };
     },

src/components/views/elements/CreateRoomButton.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';
 

src/components/views/elements/HomeButton.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';
 

src/components/views/elements/RoomDirectoryButton.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';
 

src/components/views/elements/SettingsButton.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';
 

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';