避免maskView导致的内存泄露

建议在BottomDialog的 removeMaskView() 方法中，移除maskView和异常后对maskView置空，避免内存泄露：
``` protected void removeMaskView() {
if (maskView == null) {
DialogLog.print("mask view is null");
return;
}
try {
activity.getWindowManager().removeViewImmediate(maskView);
maskView=null;
DialogLog.print("dialog remove mask view");
} catch (Throwable e) {
//...not attached to window manager
// Activity ...... has leaked window android.view.View
DialogLog.print(e);
maskView=null;
}
}