Fix IllegalStateException when closing Duck.ai (#6839)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/488551667048375/task/1211438741846582?focus=true

### Description

- Checks `supportFragmentManager.isStateSaved` once the transition has
completed to avoid `IllegalStateException`

### Steps to test this PR

- [ ] Open Duck.ai
- [ ] Close Duck.ai
- [ ] Verify that the fragment animates out and hides

Author: joshliebe

app/src/main/java/com/duckduckgo/app/browser/BrowserActivity.kt

@@ -840,9 +840,11 @@ open class BrowserActivity : DuckDuckGoActivity() {
         val fragment = duckAiFragment
         if (fragment?.isVisible == true) {
             animateDuckAiFragmentOut {
-                val transaction = supportFragmentManager.beginTransaction()
-                transaction.hide(fragment)
-                transaction.commit()
+                if (!supportFragmentManager.isStateSaved) {
+                    val transaction = supportFragmentManager.beginTransaction()
+                    transaction.hide(fragment)
+                    transaction.commit()
+                }
             }
         }
     }