こんなんググるまでもないとか、勝手に決めつけるのはよくない。

<Button title={"del"}/>

Warning: Failed prop type: The prop onPress is marked as required in Button, but its value is undefined.

はいはい、onPressがないのね。ググるまでもない。

<Button title={"del"} onPress={this.delText(index)}/>

Warning: Failed prop type: The prop onPress is marked as required in Button, but its value is undefined.

はいはい、onPressがないの??

いや、あるし。スペルミスか？？

悩むこと15分。ようやくググる

React removed "autobinding" in ES6 component classes.

The code onPress={this.submithandler} could fail because it's not bound to anything.

Solution : Bind in render()

    onPress={this.submithandler.bind( this )} 

Another Solution : Fat Arrow Class Methods

        onPress={() => this.submithandler()} 

一瞬で解決する。早くググればよかった。