Completed
Last Updated: 11 Jun 2020 15:35 by ADMIN
Release R2 2020 SP1

Workaround:

Placing the script below under the ScriptManager of the page:

kendo.ui.Chat.fn._resetToolbarButtons = function (j) {
    var k = this.wrapper.find(".k-toolbar-box");
    if (!k.is(":visible")) {
        k.show();
    }
    if (j.toolbar != false && "buttons" in j.toolbar) {
        this.options.toolbar.buttons = j.toolbar.buttons;
    }
}

Reproduction:

<telerik:RadChat runat="server" ID="RadChat1" Height="400px">
    <ClientEvents OnLoad="OnLoad"  />
</telerik:RadChat>
<script>
    function OnLoad(sender, args) {    
        sender.set_user({});
    }
</script>