Fixes compile time error for .NET 4.0
Fixes compile time error for .NET 4.0

robertmuehsig/Fluent.Ribbon,VegasoftTI/Fluent.Ribbon,donisetti/Fluent.Ribbon,wskplho/Fluent.Ribbon,dady8889/Fluent.Ribbon,fluentribbon/Fluent.Ribbon,abdelkarim/Fluent.Ribbon,punker76/Fluent.Ribbon,fmarrabal/Fluent.Ribbon,mokeev1995/Fluent.Ribbon,fluentribbon/Fluent.Ribbon,huoyaoyuan/Fluent.Ribbon,Cubey2019/Fluent.Ribbon,chrfin/Fluent.Ribbon,usmanm77/Fluent.Ribbon