Fixed DependencyProperty getters and setters (#1500)

* Fixed PopupPlacement getter and setter

* Fixed ButtonProgressAssist properties

Author: jizc

MaterialDesignThemes.Wpf/ButtonProgressAssist.cs

@@ -54,7 +54,7 @@ public static void SetIsIndeterminate(DependencyObject element, bool isIndetermi
 
         public static bool GetIsIndeterminate(DependencyObject element)
         {
-            return (bool)element.GetValue(IndicatorForegroundProperty);
+            return (bool)element.GetValue(IsIndeterminateProperty);
         }
 
         public static readonly DependencyProperty IndicatorForegroundProperty = DependencyProperty.RegisterAttached(
@@ -80,7 +80,7 @@ public static void SetIndicatorBackground(DependencyObject element, Brush indica
 
         public static Brush GetIndicatorBackground(DependencyObject element)
         {
-            return (Brush)element.GetValue(IndicatorForegroundProperty);
+            return (Brush)element.GetValue(IndicatorBackgroundProperty);
         }
 
         public static readonly DependencyProperty IsIndicatorVisibleProperty = DependencyProperty.RegisterAttached(

MaterialDesignThemes.Wpf/ValidationAssist.cs

@@ -64,12 +64,12 @@ public static void SetUsePopup(DependencyObject element, bool value)
 
         public static PlacementMode GetPopupPlacement(DependencyObject element)
         {
-            return (PlacementMode)element.GetValue(UsePopupProperty);
+            return (PlacementMode)element.GetValue(PopupPlacementProperty);
         }
 
         public static void SetPopupPlacement(DependencyObject element, PlacementMode value)
         {
-            element.SetValue(UsePopupProperty, value);
+            element.SetValue(PopupPlacementProperty, value);
         }
 
         /// <summary>