When generating a synthetic property for a Java getter/setter pair Kotlin first looks for a getter. The getter is enough to create a synthetic property with a type of the getter. On the other hand the property will not be created if only a setter presents.
When a setter comes into play property creation becomes more difficult. The reason is that the getter and the setter may have different type. Moreover, the getter and/or the setter may be overridden in a subclass.
In your case the TextView class contains a getter CharSequence getText() and a setter void setText(CharSequence). If you had a variable of type TextView your code would work fine. But you have a variable of type EditText. And the EditText class contains an overridden getter Editable getText(), which means that you can get an Editable for an EditText and set an Editable to an EditText. Therefore, Kotlin reasonably creates a synthetic property text of type Editable. The String class is not Editable, that’s why you cannot assign a String instance to the text property of the EditText class.