Error running WPF when Variable has an _

Hello i found an Issue, with the Searching for Objects from an WPF Dialog.

I used the following Line:
$tb_sqlserver = $Window.FindName("tb_sqlserver")

And on the Console the Following Error Appears:

Cannot convert the "System.Windows.Controls.TextBox" value of type "System.Windows.Controls.TextBox" to type "System.Windows.Forms.TextBox".
At H:\Powershell\Create Connection Config\Create_Connection.ps1:50 char:1
+ $tb_sqlserver = $Window.FindName("tb_sqlserver")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : MetadataError: (:) [], ArgumentTransformationMetadataException
    + FullyQualifiedErrorId : RuntimeException

When I remove the “_” from the Variable, the Error will not be Displayed.
Is that Intendet or is that an behavoir of Powershell and WPF?

Best Regards