MaterialTextField can't set text property

## 🐛 Bug Report

You can't set the text value of the MaterialTextField, it crashes the APP.
This doesn't happen in the Simulator.
Bug happening in iPhone 6S 13.3.1 and with either XF.Material 1.6.0 or 1.5.8

### Expected behavior
The text value of the MaterialTextField changes.
### Reproduction steps
Created a new blank Xamarin Forms solution.
MainPage.xaml
```

```
MainPage.xaml.cs
```
public MainPage()
{
InitializeComponent();
new Thread(new ThreadStart(() =>
{
Thread.Sleep(2000);
txt1.Text = "ofdfk";
}));
}
```

### Configuration

**Version:** 1.x

**Platform:**
- [X] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [X] :monkey: Xamarin.Forms