Throw error if callback isn't a function rather than check each time an event is received

Author: chrisbianca

lib/modules/database/index.js

@@ -176,7 +176,7 @@ export default class Database extends Base {
 
     if (this.subscriptions[handle] && this.subscriptions[handle][eventName]) {
       this.subscriptions[handle][eventName].forEach((cb) => {
-        if (isFunction(cb)) cb(new Snapshot(this, snapshot), body);
+        cb(new Snapshot(this, snapshot), body);
       });
     } else {
       FirestackDatabase.off(path, modifiersString, eventName, () => {

lib/modules/database/reference.js

@@ -96,6 +96,7 @@ export default class Reference extends ReferenceBase {
   }
 
   on(eventName: string, cb: () => any) {
+    if (!isFunction(cb)) throw new Error('The specified callback must be a function');
     const path = this._dbPath();
     const modifiers = this.query.getModifiers();
     const modifiersString = this.query.getModifiersString();