[FIX] incomplete tests not deleted properly

Ensure tests are deleted from state instead of stored as undefined

src/History.js

@@ -130,7 +130,7 @@ export default class History extends Component {
 				let newState = {
 					progressHistoryIncomplete: this.state.progressHistoryIncomplete,
 				};
-				delete newState.progressHistoryIncomplete[progressIndex];
+				newState.progressHistoryIncomplete.splice(progressIndex, 1);
 				this.setState(newState);
 			});
 	}