Throw an error when header is invalid

CHANGELOG.md

@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+- Throw error in development mode when header navigation option is set to a string - a common mistake that would otherwise result in a cryptic error message.
 
 ## [2.5.4] - [2018-06-27](https://github.com/react-navigation/react-navigation/releases/tag/2.5.4)
 ### Fixed

src/views/StackView/StackViewLayout.js

@@ -120,6 +120,12 @@ class StackViewLayout extends React.Component {
     const { options } = scene.descriptor;
     const { header } = options;
 
+    if (__DEV__ && typeof header === 'string') {
+      throw new Error(
+        `Invalid header value: "${header}". The header option must be a valid React component or null, not a string.`
+      );
+    }
+
     if (header === null && headerMode === 'screen') {
       return null;
     }