After maternity leave, I can't get fired, because my employer...

You need to send an application for voluntary dismissal by registered letter with a notice of delivery and a description of the attachment to the company's legal address (if you have complied with all the listed requirements, then the application indicated by you will be suitable). If, as a result, the application will be left without consideration, then in this case you should apply to court. After all, this is a violation of Article 43 of the Constitution of Ukraine, which prohibits forced labor and provides an opportunity for everyone to earn a living by work that he freely chooses or freely agrees to.