Made callback for isPickerShow optional (#296)

index.d.ts

@@ -266,5 +266,5 @@ export default class Picker {
      *
      * @memberof Picker
      */
-    static isPickerShow(): boolean
+    static isPickerShow(fn?: (err: any, message: any) => void): boolean
 }

index.js

@@ -91,7 +91,7 @@ export default {
             else if(ios){
                 returnValue = !err;
             }
-            fn(returnValue);
+            fn && fn(returnValue);
         });
     }
 };