It means somewhere in your chain of calls, you tried to access a Property or call a method on an object that was null.
Given your statement:
img1.ImageUrl = ConfigurationManager
.AppSettings
.Get("Url")
.Replace("###", randomString)
+ Server.UrlEncode(
((System.Web.UI.MobileControls.Form)Page
.FindControl("mobileForm"))
.Title);
I'm guessing either the call to AppSettings.Get("Url") is returning null because the value isn't found or the call to Page.FindControl("mobileForm") is returning null because the control isn't found.
You could easily break this out into multiple statements to solve the problem:
var configUrl = ConfigurationManager.AppSettings.Get("Url");
var mobileFormControl = Page.FindControl("mobileForm")
as System.Web.UI.MobileControls.Form;
if(configUrl != null && mobileFormControl != null)
{
img1.ImageUrl = configUrl.Replace("###", randomString) + mobileControl.Title;
}