If Checkbox is true then sending mail to user's mail id

Visualforce Page:

<apex:page controller="sendemailcls">
    <apex:form >
        <apex:pageBlock >
            <apex:actionFunction name="chang" action="{!sendmail}"/>
            <apex:inputCheckbox value="{!checkingbox}" onchange="chang()"/>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Apex Class:


public with sharing class sendemailcls {
    String Email;
    public PageReference sendmail() {
        if(checkingbox==true){
           
            Messaging.SingleEmailMessage msem= new Messaging.SingleEmailMessage();
            msem.setToAddresses(new string[] {Email});
            msem.setSubject('your input checkbox was checked');
            msem.setHTMLBody('<H1>This mail from salesforce</H1>');
            System.debug('------------------------------'+msem);
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] {msem});
                              }
        return null;
    }


    public boolean checkingbox { get; set; }
    public sendemailcls(){
       String Userid= UserInfo.getUserid();
       User u=[select id,email from User where id=:Userid];
       Email=u.email;
                         }
}