Made sure that we don't try to fetch IDs that aren't in the database.

src/components/ViewSightings.js

@@ -154,7 +154,13 @@ class ViewSightings extends React.Component {
         e.preventDefault();
         const itemSighting = firebase.database().ref("sightings/" + this.state.id);
         itemSighting.once("value").then((snapshot) => {
+            // Die if there's no data for that ID.
+            if (!snapshot.exists()) {
+                return;
+            }
+            
             let data = snapshot.val();
+
             this.setState({
                 date: data.date,
                 time: data.time,