fix: custom url scheme is mandatory (#3)

README.md

@@ -21,15 +21,17 @@ Share intent in a native module for Expo (React Native).
 
 ## Installation
 
-Install npm package
+**Install npm package**
 
 ```bash
 yarn add expo-share-intent
 # or
 npm install expo-share-intent
 ```
 
-add expo plugin into your `app.json`
+**Into your `app.json`:**
+
+- add expo plugin
 
 ```json
   "plugins": [
@@ -39,6 +41,14 @@ add expo plugin into your `app.json`
 
 > by default only text and url sharing is activated
 
+- configure a custom URL scheme
+
+```json
+  "scheme": "my-app"
+```
+
+> More info here : [Linking to your app](https://docs.expo.dev/guides/linking/#linking-to-your-app)
+
 ## Usage
 
 #### Use the hook in your App

plugin/src/ios/writeIosShareExtensionFiles.ts

@@ -157,6 +157,11 @@ export function getShareExtensionViewControllerContent(
   console.debug(
     `[expo-share-intent] add ios share extension (scheme:${scheme} appIdentifier:${appIdentifier})`
   );
+  if (!scheme) {
+    throw new Error(
+      "[expo-share-intent] missing custom URL scheme 'expo.scheme' in app.json ! (see https://docs.expo.dev/guides/linking/#linking-to-your-app)"
+    );
+  }
 
   const content = fs.readFileSync(
     path.resolve(__dirname, "./ShareExtensionViewController.swift"),