assertIsPrepared() being called randomly

**Problem Description:**
My app would randomly crash due to assertIsPrepared() being called. Is this a known bug or am I doing something wrong?

```
fileprivate func assertIsPrepared() {
precondition(isPrepared, "You must call the `prepare` function before interacting with the bulletin.")
}
```

**Steps to reproduce:**
This is what I call when a button is pressed:

```
bulletinManager.backgroundViewStyle = .none
bulletinManager.backgroundColor = UIColor(red:0.30, green:0.69, blue:0.94, alpha:1.00)
bulletinManager.showBulletin(above: self)
```

The crash seems random.

**Environment:**
- Device: N/A
- OS: iOS12
- Version of BulletinBoard: 2.0.2