I created a product with three variants and selected one of the variants, then clicked on Add to Chart. However, when module redirects to cart page i received and error about Tax Rates.

Detailed error logged under Admin > Event viewer is:

ActiveTabName: Buy Now
RawURL: /ShoppingCart/BuyNow/tabid/78/ctl/Cart/mid/442/Default.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.*.com/ShoppingCart/ItemDetails/tabid/77/ProductID/1/Default.aspx
UserAgent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: a2ad0275-7840-4809-b7c1-5a9f1623aded
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: Smith.DNN.Modules.BuyNow.Cart.GetTaxRate
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Smith.DNN.Modules.BuyNow.Cart.GetTaxRate() at Smith.DNN.Modules.BuyNow.Cart.GetTax() at Smith.DNN.Modules.BuyNow.Cart.GetTotalAfterTax() at Smith.DNN.Modules.BuyNow.Cart.UpdateTotalLabel() at Smith.DNN.Modules.BuyNow.Cart.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source:
Server Name: WIN-*****

Can someone help me troubleshoot Unable to cast object of type 'System.Web.UI.WebControls.LinkButton' to type 'System.Web.UI.WebControls.ImageButton' error?

AssemblyVersion: 5.6.2
PortalID: 0
PortalName: *
UserID: 3
UserName: *
ActiveTabID: 76
ActiveTabName: Boutique
RawURL: /Boutique.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://*.com/Default.aspx?tabid=76&error=Object+reference+not+set+to+an+instance+of+an+object.&content=0
UserAgent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.0; Trident/5.0; chromeframe/13.0.782.107)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 3a93476a-6052-4c47-afce-047eb87fecc0
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: Smith.DNN.Modules.CategoryMenu.CategoryMenu.Page_Load
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Smith.DNN.Modules.CategoryMenu.CategoryMenu.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source:
Server Name: *******