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

Start by running the linked BugMaterialDesignToggleButton example with MaterialDesignThemes 3.2.0 and clicking a ToggleButton with IsThreeState enabled. Trace the ToggleButton behavior around the IndeterminateCheck lookup and reproduce the InvalidOperationException. Done means three-state clicking no longer throws and the indeterminate state works.

Written by the indexing model from the issue text.