The expression return $this->bestBidPrice could return the type null which is incompatible with the type-hinted return double. Consider adding an additional type-check to rule them out.
The expression return $this->bidQuantity could return the type null which is incompatible with the type-hinted return double. Consider adding an additional type-check to rule them out.
The expression return $this->bestAskPrice could return the type null which is incompatible with the type-hinted return double. Consider adding an additional type-check to rule them out.
The expression return $this->askQuantity could return the type null which is incompatible with the type-hinted return double. Consider adding an additional type-check to rule them out.