Reproduce:

1. navigate to a Wikidata Item.
2. Go to a statementgroup, click edit
3. Click Add reference (by accident or in the intent to add a reference but turn away from it because you don't know how to use it)
4. Do nothing with the reference property field; you can do something else, like changing the value etc.

Result:
Save can't be clicked even though the user may made actual changes, seemingly because the Reference property is empty.
This leads to potential loss of changes (since the user may have changed something else) if the user does not find the cause of the blocked save button.

What should happen:

• If there is nothing in the property, the statement can still be saved (It should behave like "add qualifier" behaves after a click: )
• And/Or there should be a notification why save is not possible to click/execute; highlighting the "corrupt" input field may also help.

Update

• Initially, I assumed and checked that the qualifier behavior is different and wrote »It should behave like "add qualifier" behaves after a click: If there is nothing in the property, the statement can still be saved.«. However, on a second check I could not reproduce it anymore. Possibly another bug.