Refactor method for calculating height

app/src/main/java/ch/nuth/zhaw/exbox/SortedBinaryTree.java

@@ -74,14 +74,19 @@ public class SortedBinaryTree<T extends Comparable<T>> implements Tree<T> {
     }
 
     protected int calcHeight(TreeNode<T> node) {
-        int result = 0;
+        if (node == null) {
+            return 0;
+        } else {
+            int result = 1;
 
             if (node.left != null || node.right != null) {
-            result = Stream.of(node.left, node.right).filter(
-                (innerNode) -> innerNode != null).mapToInt((innerNode) -> calcHeight(innerNode)).max().getAsInt();
+                result += Stream.of(node.left, node.right).filter(
+                        (innerNode) -> innerNode != null).mapToInt((innerNode) -> calcHeight(innerNode)).max()
+                        .getAsInt();
             }
 
-        return result + 1;
+            return result;
+        }
     }
 
     public int height() {