fix: unwrap on root when tree is persisted

src/tree.rs

@@ -89,15 +89,22 @@ where
         // Load existing db instance
         let db = D::load(db_config)?;
 
-        // Load root
-        let root = H::deserialize(db.get(Key(0, 0).into())?.unwrap());
+        // Load root 
+        let root = match db.get(Key(0, 0).into())? {
+            Some(root) => H::deserialize(root),
+            None => H::default_leaf(),
+        };
 
         // Load depth & next_index values from db
-        let depth = db.get(DEPTH_KEY)?.unwrap().try_into().unwrap();
-        let depth = usize::from_be_bytes(depth);
-
-        let next_index = db.get(NEXT_INDEX_KEY)?.unwrap().try_into().unwrap();
-        let next_index = usize::from_be_bytes(next_index);
+        let depth = match db.get(DEPTH_KEY)? {
+            Some(depth) => usize::from_be_bytes(depth.try_into().unwrap()),
+            None => 20
+        };
+
+        let next_index = match db.get(NEXT_INDEX_KEY)? {
+            Some(next_index) => usize::from_be_bytes(next_index.try_into().unwrap()),
+            None => 0,
+        };
 
         // Load cache vec
         let mut cache = vec![H::default_leaf(); depth + 1];