Select default value for renderer

src/components/inputs/StringInput.jsx

@@ -3,7 +3,7 @@ import input from '../../config/input.js'
 
 class StringInput extends React.Component {
   static propTypes = {
-    value: React.PropTypes.string.isRequired,
+    value: React.PropTypes.string,
     style: React.PropTypes.object,
     onChange: React.PropTypes.func.isRequired,
   }

src/components/modals/SettingsModal.jsx

@@ -68,7 +68,7 @@ class SettingsModal extends React.Component {
             ['mbgljs', 'MapboxGL JS'],
             ['ol3', 'Open Layers 3']
           ]}
-          value={(this.props.mapStyle.metadata || {})['maputnik:renderer']}
+          value={(this.props.mapStyle.metadata || {})['maputnik:renderer'] || 'mbgljs'}
           onChange={this.onRendererChange.bind(this)}
         />
       </InputBlock>