Fix MetroProgressBar System.Windows.Media.Animation Warning 6

this pr should fix this warning. it happens on initial show and if we toggle `IsIndeterminate`.

```
System.Windows.Media.Animation Warning: 6 : Unable to perform action because the specified Storyboard was never applied to this object for interactive control.; Action='Remove'; Storyboard='System.Windows.Media.Animation.Storyboard'; Storyboard.HashCode='12083158'; Storyboard.Type='System.Windows.Media.Animation.Storyboard'; TargetElement='System.Windows.Media.Animation.Storyboard'; TargetElement.HashCode='12083158'; TargetElement.Type='System.Windows.Media.Animation.Storyboard'
```

Closes #1631