Prevent modifying the document once saved

@@ -7,6 +7,7 @@
 - Implement PdfPageLabels
 - Typo: rename "litteral" with "literal"
 - Fix tabs and other spaces placeholder
+- Prevent modifying the document once saved
 
 ## 3.7.4
 

@@ -114,11 +114,13 @@ class Document {
   bool _paint = false;
 
   void addPage(Page page, {int? index}) {
+    assert(!_paint, 'The document has already been saved.');
     page.generate(this, index: index);
     _pages.add(page);
   }
 
   void editPage(int index, Page page) {
+    assert(!_paint, 'The document has already been saved.');
     page.generate(this, index: index, insert: false);
     _pages.add(page);
   }