Thank you for posting your query.
Firstly, Congratulations on making a big decision in your professional life. Not many people in Dubai, UAE are in a position to make such decisions, due to the following facts:
1. Comfort with your current financial situation.
2. Waiting for the bonus.
3. Not skilled enough to take on a challenge elsewhere.
4. Have too many loans or liabilities and thus cannot leave their current position.
5. Visa and NOC issues.
Considering the fact that you have overcome all such above mentioned nuances, you need to reflect of why you were served the warning letter in the first instance.
1. If it is a disciplinary issue that was highlighted, then you should be apologetic and ensure your behaviour does not offend or stray away from the policies and procedures of the organization.
2. Your manager does not like you and for some reason has a habit of issuing warning letters. In such a case smile and accept it. You have decided to be part of the organization. Don't like the way they operate or their Corporate guidelines does not match your philosophy, then resign. No one has forced your decision. What's the point in crying around.
Answer to your question
Yes. The organization has every right legally to issue a warning letter.
Well, you need to look at the time frame and the change in your behaviour since you have tendered in your resignation.
1. Have you been praised by colleagues of making the right decision? Well, if your decision is right, why have they not resigned with you?
2. Have you resigned on a poor note or conflict of interest with your organization? Dont like them. No problem, move on.....If you feel you are Mr. Right, then you should be the Board member.
3. You have resigned and have become more vocal and critical in your daily comments regarding the working conditions and the management? You have resigned because it is your decision. Who has given you the right to defame or talk bad.
You need to understand that on legal grounds, you may be impacting or influencing other employees behaviour and productivity and if you are making poor comments and statements, then you surely are looking for a mandatory ban enforced on you legally if proven guilty.
You have resigned, move on. Is it a career you want to build or look like a super hero in front of a few nut cases at work who will never be part of your life in the future.
Is it that your employer gave you a verbal warning earlier and now wants to document it via a warning letter? OR
Is the Company issuing you a second warning?
What exactly does the warning letter state? - Usually when a warning letter is issued some corrective action is expected from the employee. Will the corrective action be practical in your case? Since you are on notice do you have sufficient time and opportunity to exhibit the improvement(s)?
Also, very important, please try figure out the motive of the Co. behind issuing the letter. Will it affect your Relieving, Settlement, Future Prospects, etc. in any way?
Legally Company can to issue warning letters during notice period but, from my experience, such letters are more related to casual behaviour, gossip, spreading negativity etc. and not for performance.
Take a constructive approach figure out what exactly the Co. wants and why. If it's not going to have any impact on your future then don't spend time arguing over it.
Hope this helps.