Fix 1405 handle android back button in composer (#1446)

* handle android back button in composer

* improve backHandler error handling

* simplify composer onClose functionality

src/lib/routes/back-handler.ts

@@ -1,8 +1,19 @@
+import {isAndroid} from 'platform/detection'
 import {BackHandler} from 'react-native'
 import {RootStoreModel} from 'state/index'
 
 export function init(store: RootStoreModel) {
-  BackHandler.addEventListener('hardwareBackPress', () => {
-    return store.shell.closeAnyActiveElement()
-  })
+  // only register back handler on android, otherwise it throws an error
+  if (isAndroid) {
+    const backHandler = BackHandler.addEventListener(
+      'hardwareBackPress',
+      () => {
+        return store.shell.closeAnyActiveElement()
+      },
+    )
+    return () => {
+      backHandler.remove()
+    }
+  }
+  return () => {}
 }