I have encountered this before, and it was because I had two FloatingAction buttons on one screen, I had to add a heroTag property + value per FloatingActionButton in order for the error to go away.

Example:

new FloatingActionButton(
    heroTag: "btn1",
    ...
)

new FloatingActionButton(
    heroTag: "btn2",
    ...
)

From the example code you provided it doesn't appear that you have a FloatingActionButton, but from the error it does seem to reference it:

I/flutter (21786): In this case, multiple heroes had the following tag: default FloatingActionButton tag

Perhaps you used it on the page you were navigating to which then triggered the error. Note that if you're using a programmatic way of creating tagged heroes, you will need to find a way of giving them different tags. For example, if you have a ListView.builder() creating FloatingActionButtons, try passing tags with string formatting so each button has a different tag, e.g.: heroTag: "btn$index".

In any event, hope that helps someone.