"Nullable object must have a value" error when removing a tag in XBRL.

When trying to remove a tag in XBRL you receive the following error:
Type: InvalidOperationException
Message: Nullable object must have a value.
Stack Trace: at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)

at Clarity.FSR.DomainModel.XBRL.XbrlStagingArea.CreateElements(IEnumerable`1 selectedCells, IEnumerable`1 badNames)
at Clarity.FSR.DomainModel.XBRL.XbrlStagingArea.LinkInfosCreated(IEnumerable`1 selectedCells, IEnumerable`1 badNames)
at Clarity.FSR.DomainModel.XBRL.XbrlSuperviser.LinkInfosCreated(IEnumerable`1 linkInfo, IEnumerable`1 badNames)
at Clarity.FSR.UI.Forms.LayoutForm.OfficeObj_CreateLinkInfo(Object sender, LinkInfoEventArgs arg)
at Clarity.FSR.DomainModel.XBRL.XbrlStagingArea.RaiseCreateLinkInfo(String enumeratedVal)
at Clarity.FSR.DomainModel.XBRL.XbrlStagingArea.AssignTagInfo(String enumeratedVal)
at Clarity.FSR.DomainModel.XBRL.XbrlInspectorController.AssignTagToCell()
at Clarity.FSR.DomainModel.XBRL.XbrlInspectorController.AssignmentLogic()
at Clarity.FSR.UI.Controls.XBRL.XbrlInspectorView.buttonAssign_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

This issue is caused by the fact that the tag doesn't exist and it's only duplicated in the UI.

FSR 6.6.0.454 up to 6.6.1.165 (FSR 6.6 with Fix Pack 1 and Hot Fix 17)

In order to replicate the issue, please follow these steps:

1. Load the XBRL interface in an Excel object.

2. Select a tag;

3. Select the cell where you want to apply the tag;

4. Click Apply.

After step 4 you will notice that the tag appears twice in the XBRL previewer.

*This issue only occurs for some tags and it's intermittent.

Clicking save or browsing to another cell and then browsing back to the cell will solve the issue. The viewer will be refreshed and it will only show one tag.