Icon doesn't render for the first tab inside a scene

### Version
Tell us which versions you are using:

- react-native-router-flux v4.?.?
- react v16.?.?
- react-native v0.?.?

### Expected behaviour
I used custom component to render icons on the tab, tab icons should work on all tabs but not working for the first one

### Actual behaviour
Tab icon should render on all tabs

### Steps to reproduce
For non-obvious bugs, please fork this component, modify Example project to reproduce your issue and include link here.

Tab icon doesn't show for Component A
### Reproducible Demo

Please provide a minimized reproducible demonstration of the problem you're reporting.

Issues that come with minimal repro's are resolved much more quickly than issues where a maintainer has to reproduce themselves.