not able to use backHandler in side menu screens in react-native-router-flux.

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

- react-native-router-flux v^3.39.2
- react v16.4.2
- react-native v0.56.0

### Expected behaviour

i want to go back to home screen when android back button is pressed in side menu screens.

### Actual behaviour

after navigating to screens in side menu when i press android back button app exits or back button is not working.

### Steps to reproduce
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### Reproducible Demo

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

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