I have created a report parameter that accepts null as a value(AllowNull=True) and have set null as the default value by using the expression =Null on the Value property of the report parameter.

This report parameter also has a data source assigned to it so it has available values. Here is a look at the full setup:

When I call the "getReportParameters()" function on the HTML5 Report Viewer, there is an exception thrown in the console that looks as follows:

telerikReportViewer:1 Error: The available values of parameter Parameter1 do not contain Value property that equals null
    at Ae (telerikReportViewer:1:45529)
    at Object.getReportParameters (telerikReportViewer:1:38265)
    at Object.getReportParameters (telerikReportViewer:1:108469)
    at <anonymous>:1:50