Add animators null judgment

Author: HarlonWang

library/src/main/java/com/wang/avi/Indicator.java

@@ -106,6 +106,9 @@ private void startAnimators() {
     }
 
     private void stopAnimators() {
+        if (mAnimators==null){
+            return;
+        }
         for (ValueAnimator animator : mAnimators) {
             if (animator != null && animator.isStarted()) {
                 animator.removeAllUpdateListeners();