If you have recently upgraded GlassMapper you might run into an issue where GetRenderingParameters returns null. I believe this started occuring in version 184.108.40.206 or 220.127.116.11. I was only experiencing this in WebForms. The issue occurs when you call GetRenderingParameters as described in this article (section “ASP .NET Webforms”).
The good news is that this issue has already been reported so hopefully it will be fixed in the next release. If you really need to upgrade to a version with this issue then you can easily fix this by downloading the code from github here and compiling Glass.Mapper.Sc DLL yourself.
The only code change that is required is to change
var sublayout = _control as Sublayout;
to remove underscore in control to reference the method parameter
var sublayout = control as Sublayout;
On line 40 of GetRenderingParametersInternal in Glass.Mapper.Sc.Web.Ui.GetRenderingParametersInternal see file in Github here