I think that has been true for AST editors because they remain halfway in the traditional text-editing paradigm, which involves these invalid states because the granularity of the interface is single character operations, while the model you are specifying with those characters (the AST) doesn't recognize those units.

An alternative is to have the editor operate on language constructs rather than characters. Rather than parsing the program view in order to arrive at a model of your program, you arrive at a view by rendering the model in some way. But the key thing is the model always stays intact because the operations you perform on it through your editor are in terms of language constructs (e.g. a single action in your editor might be 'add property' or 'create class', which is ordinarily accomplished by typing out lots of characters that can hopefully be parsed into such things).