Fix useState TypeScript errors

src/components/alphaPicker/AlphaPickerComponent.tsx

@@ -9,7 +9,7 @@ type AlphaPickerProps = {
 // React compatibility wrapper component for alphaPicker.js
 // eslint-disable-next-line @typescript-eslint/no-empty-function
 const AlphaPickerComponent: FunctionComponent<AlphaPickerProps> = ({ onAlphaPicked = () => {} }: AlphaPickerProps) => {
-    const [ alphaPicker, setAlphaPicker ] = useState(null);
+    const [ alphaPicker, setAlphaPicker ] = useState<AlphaPicker>();
     const element = useRef<HTMLDivElement>(null);
 
     useEffect(() => {