feat(error-handling): Improve config error handling

see #51

Author: hypery2k

README.md

@@ -126,6 +126,14 @@ The android:launchMode="singleTask" tells the Android operating system to launch
 
 # FAQ
 
+## Callback handling
+
+The "handleOpenURL" callback must be called before application initialization, otherwise you'll see this error in the console:
+
+```
+    No callback provided. Please ensure that you called "handleOpenURL" during application init!
+``` 
+
 ## Webpack
 
 ### TypeScript Config

urlhandler.common.ts

@@ -36,5 +36,10 @@ export function handleOpenURL(handler: UrlHandlerCallback): void {
 }
 
 export function getCallback(): UrlHandlerCallback {
+    if (!URL_HANDLER_CB) {
+        URL_HANDLER_CB = function () {
+            console.error('No callback provided. Please ensure that you called "handleOpenURL" during application init!');
+        };
+    }
     return URL_HANDLER_CB;
 }