WPF Error: "Property" must have a non-null value.

This problem generally shows that the type specified in triggers is now nullable.
The workaround is to replace the trigger with DataTrigger and then convert the Binding+converter to a verbose non-null value.
Analogy: <style x:key= "Disablestyle" targettype= "button" >
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<condition binding= "{Binding elementname=ckb1, path=ischecked, Converter={staticresource Nullabletobooleanconverter}} "Value=" true "></Condition>
<condition binding= "{Binding elementname=ckb2, path=ischecked, Converter={staticresource Nullabletobooleanconverter}} "Value=" true "></Condition>
</MultiDataTrigger.Conditions>
<setter property= "isenabled" value= "False" ></Setter>
</MultiDataTrigger>
</Style.Triggers>
</Style>


Nullabletobooleanconverter: PublicObject Convert(Objectvalue,Type TargetType,Objectparameter,CultureInfo Culture)        {           Nullable<BOOL>Ischeck=value as Nullable<BOOL>;           if (NULL==Ischeck)            {               returnfalse;            }           Else            {               return Ischeck.Value;            }        }

WPF Error: "Property" must have a non-null value.