ToggleButton throws exception if IsThreeState is enabled

The ToggleButton throws an exception if the "IsThreeState" property is set to true and the button is clicked.

Exception thrown: 'System.InvalidOperationException' in PresentationFramework.dll
An unhandled exception of type 'System.InvalidOperationException' occurred in PresentationFramework.dll
Der Name "IndeterminateCheck" wurde im Namensbereich von "System.Windows.Controls.Grid" nicht gefunden.

Translated it would probably s.th. like: The name "IndeterminateCheck" could not be found in the namespace of "System.Windows.Controls.Grid"

Versions:
MaterialDesignThemes: 3.2.0
MaterialDesignColors: 1.2.7

Example Repo: https://github.com/badcel/BugMaterialDesignToggleButton.git

首先使用 MaterialDesignThemes 3.2.0 运行链接的 BugMaterialDesignToggleButton 示例，并点击启用了 IsThreeState 的 ToggleButton。跟踪 IndeterminateCheck 查找附近的 ToggleButton 行为，并重现 InvalidOperationException。当三态点击不再抛出异常且不确定状态能够正常工作时，即表示完成。