Fixed issue #16 where if custom button image is missing, there will be an exception that prevents ThemeableBrowser from launching.
Fixed issue #16 where if custom button image is missing, there will be an exception that prevents ThemeableBrowser from launching.

akriger/cordova-plugin-themeablebrowser-,akriger/cordova-plugin-themeablebrowser-,akriger/cordova-plugin-themeablebrowser-,knyazz/cordova-plugin-themeablebrowser,archriss/inappbrowser-android,zachawilson/cordova-plugin-themeablebrowser,tomsean/cordova-plugin-themeablebrowser,imskojs/cordova-plugin-themeablebrowser,JStumpp/cordova-plugin-themeablebrowser,initialxy/cordova-plugin-themeablebrowser,knyazz/cordova-plugin-themeablebrowser,knyazz/cordova-plugin-themeablebrowser,cbartlett/cordova-plugin-themeablebrowser,zachawilson/cordova-plugin-themeablebrowser,initialxy/cordova-plugin-themeablebrowser,initialxy/cordova-plugin-themeablebrowser,imskojs/cordova-plugin-themeablebrowser,ThinklabsGbR/cordova-plugin-themeablebrowser,cbartlett/cordova-plugin-themeablebrowser,imskojs/cordova-plugin-themeablebrowser,imskojs/cordova-plugin-themeablebrowser,knyazz/cordova-plugin-themeablebrowser,archriss/inappbrowser-android,tomsean/cordova-plugin-themeablebrowser,ThinklabsGbR/cordova-plugin-themeablebrowser,cbartlett/cordova-plugin-themeablebrowser,zachawilson/cordova-plugin-themeablebrowser,archriss/inappbrowser-android,JStumpp/cordova-plugin-themeablebrowser,ThinklabsGbR/cordova-plugin-themeablebrowser,archriss/inappbrowser-android,cbartlett/cordova-plugin-themeablebrowser,akriger/cordova-plugin-themeablebrowser-,archriss/inappbrowser-android,tomsean/cordova-plugin-themeablebrowser,JStumpp/cordova-plugin-themeablebrowser,tomsean/cordova-plugin-themeablebrowser,zachawilson/cordova-plugin-themeablebrowser,akriger/cordova-plugin-themeablebrowser-,initialxy/cordova-plugin-themeablebrowser,knyazz/cordova-plugin-themeablebrowser,tomsean/cordova-plugin-themeablebrowser,ThinklabsGbR/cordova-plugin-themeablebrowser,zachawilson/cordova-plugin-themeablebrowser,cbartlett/cordova-plugin-themeablebrowser,imskojs/cordova-plugin-themeablebrowser,JStumpp/cordova-plugin-themeablebrowser,JStumpp/cordova-plugin-themeablebrowser,initialxy/cordova-plugin-themeablebrowser,ThinklabsGbR/cordova-plugin-themeablebrowser