Cant write/read to/from external storage with LibGDX

My create method looks like this:

@Override
public void create () {
    batch = new SpriteBatch();

    FileHandle file = Gdx.files.external("file.txt");
    file.writeString("My god, it's full of stars", false);
}

I allso included:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

The Exception i get is this:

02-13 14:45:51.858 12439-12466/com.snowdevs.tweetiebirds E/AndroidRuntime: FATAL EXCEPTION: GLThread 1120
Process: com.snowdevs.tweetiebirds, PID: 12439
com.badlogic.gdx.utils.GdxRuntimeException: Error writing file: file.txt (External)
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:353)
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339)
   at com.snowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22)
   at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254)
   at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519)
   at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240)
Caused by: com.badlogic.gdx.utils.GdxRuntimeException: Error writing file: file.txt (External)
   at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:330)
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350)
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
   at com.snowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
   at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
   at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
   at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 
Caused by: java.io.FileNotFoundException: /storage/emulated/0/file.txt: open failed: EACCES (Permission denied)
   at libcore.io.IoBridge.open(IoBridge.java:452)
   at java.io.FileOutputStream.<init>(FileOutputStream.java:87)
   at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:322)
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350) 
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
   at com.snowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
   at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
   at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
   at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 
Caused by: android.system.ErrnoException: open failed: EACCES (Permission denied)
   at libcore.io.Posix.open(Native Method)
   at libcore.io.BlockGuardOs.open(BlockGuardOs.java:186)
   at libcore.io.IoBridge.open(IoBridge.java:438)
   at java.io.FileOutputStream.<init>(FileOutputStream.java:87) 
   at com.badlogic.gdx.files.FileHandle.writer(FileHandle.java:322) 
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:350) 
   at com.badlogic.gdx.files.FileHandle.writeString(FileHandle.java:339) 
   at com.snowdevs.tweetiebirds.TweetieBirdsGame.create(TweetieBirdsGame.java:22) 
   at com.badlogic.gdx.backends.android.AndroidGraphics.onSurfaceChanged(AndroidGraphics.java:254) 
   at android.opengl.GLSurfaceView$GLThread.guardedRun(GLSurfaceView.java:1519) 
   at android.opengl.GLSurfaceView$GLThread.run(GLSurfaceView.java:1240) 

I use a Nexus 5 with Android 6 Marshmallow for testing.
I searched and found that Android 6 uses runtime permissions but the LibGDX guys says that it works even on Android 6… Is there a fix With or Without runtime permissions?