Fix mismatched parentheses causing exceptions.

src/main/java/org/nwapw/abacus/parsing/ShuntingYardParser.java

@@ -162,6 +162,7 @@ public class ShuntingYardParser implements Parser<Match<TokenType>>, PluginListe
     @Override
     public TreeNode constructTree(List<Match<TokenType>> tokens) {
         tokens = intoPostfix(new ArrayList<>(tokens));
+        if(tokens == null) return null;
         Collections.reverse(tokens);
         return constructRecursive(tokens);
     }