change active index view does not allow js function to work?

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ASP.NET

Javascript

I have a multi view control with 2 view.
There are one textbox in each view, that I want to check them not to be empty with java script function.

function check() 
{
      if (freetextboxEmpty() && textboxEmpty()) {
           if (confirmation()) {
                return true;
           }
      }                        

      return false;
}

function freetextboxEmpty() 
{
      var x = document.getElementById('<%=FreeTextBox1.ClientID%>').value;

      if (x == '')
          return false;
      else
          return true;
}

function textboxEmpty() 
{
      var x = document.getElementById('<%=Sub_txt.ClientID%>').value;

      if (x == '')
          return false;
      else
          return true;
}

I put the check function in OnClientClick event of button.

freetextboxEmpty() function check the emptyness of textbox in view 1 and textboxEmpty() function check the textbox in view 2.

My problem is when I am in view 1 and press button, js function check the first textbox and not the second textbox in view 2 and when I am in view 2, js function does not work.

<asp:Button ID="Save_btn" 
            runat="server" 
            Text="save"  
            CssClass="Initial" 
            onclick="Save_btn_Click" 
            Width="100px" 
            OnClientClick="return check();" />

<asp:MultiView ID="MainView1" runat="server">        
       <asp:View ID="View11" runat="server"> 
              <asp:TextBox ID="Sub_txt" 
                           runat="server" 
                           Font-Size="Larger" 
                           Height="35px" 
                           Width="95%"></asp:TextBox>
       </asp:View>  
       
       <asp:View ID="View12" runat="server"> 
              <asp:TextBox ID="FreeTextBox1" runat="Server"/>
       </asp:View>              
</asp:MultiView>