html - vb.net Object reference not set to an instance of an object error when clicking a button on a webbrowser -

im trying click button in webbrowser.

i have created button in form click button, gives me error on line

    dim elements htmlelementcollection = custombrowser.document.all 

could website blocking me clicking button?? here custom browser class:

public class custombrowser   inherits webbrowser public sub new()     me.scripterrorssuppressed = true end sub private sub docomplited() handles me.documentcompleted     dim tp tabpage = me.tag        my.settings.ddd = tp.text     if me.documenttitle.length > 15         tp.text = me.documenttitle.substring(0, 14) & ""     else         tp.text = me.documenttitle       end if     my.settings.ddd = tp.text   end sub  private sub custombrowser_locationchanged(byval sender object, byval e system.eventargs) handles me.locationchanged  end sub   private sub custombrowser_navigated(byval sender object, byval e system.windows.forms.webbrowsernavigatedeventargs) handles me.navigated     form1.label1.text = me.url.tostring     end sub end class 

the buttton code :

private sub button2_click(byval sender system.object, byval e system.eventargs) handles button2.click       if custombrowser.document isnot nothing         dim elements htmlelementcollection = custombrowser.document.all            each webpageelement htmlelement in elements             custombrowser.document.all("click").invokemember("click")             if webpageelement.getattribute("class") = "click"                  webpageelement.invokemember("click")             end if           next     end if end sub