Fix parser generation exception caused by unmodifiable collections usages from com.intellij.util.containers.ContainerUtil (#389)

Author: dtsaryov

src/org/intellij/grammar/analysis/BnfFirstNextAnalyzer.java

@@ -429,15 +429,15 @@ else if (GrammarUtil.isExternalReference(expression)) {
   private static @NotNull Set<BnfExpression> exprSetIntersection(@NotNull Set<BnfExpression> a, @NotNull Set<BnfExpression> b) {
     Set<BnfExpression> filter = newExprSet(a);
     filter.retainAll(newExprSet(b));
-    Set<BnfExpression> result = union(a, b);
+    Set<BnfExpression> result = new HashSet<>(union(a, b));
     result.retainAll(filter);
     return result;
   }
 
   private static @NotNull Set<BnfExpression> exprSetDifference(@NotNull Set<BnfExpression> a, @NotNull Set<BnfExpression> b) {
     Set<BnfExpression> filter = newExprSet(a);
     filter.removeAll(newExprSet(b));
-    Set<BnfExpression> result = union(a, b);
+    Set<BnfExpression> result = new HashSet<>(union(a, b));
     result.retainAll(filter);
     return result;
   }

src/org/intellij/grammar/diagram/BnfDiagramProvider.java

@@ -107,9 +107,11 @@ public String getElementTitle(BnfRule o) {
     @Override
     public Object @NotNull [] getNodeItems(BnfRule element) {
       Map<PsiElement, RuleGraphHelper.Cardinality> map = myGraphHelper.getFor(element);
-      List<Item> entries = ContainerUtil.mapNotNull(map.entrySet(), p -> p.getKey() instanceof BnfRule o ? new Item(o, p.getValue()) : null);
-      Collections.sort(entries, (o, o1) -> Comparing.compare(o.rule.getName(), o1.rule.getName()));
-      return entries.toArray();
+      return map.entrySet().stream()
+        .map(p -> p.getKey() instanceof BnfRule o ? new Item(o, p.getValue()) : null)
+        .filter(Objects::nonNull)
+        .sorted((o, o1) -> Comparing.compare(o.rule.getName(), o1.rule.getName()))
+        .toArray();
     }
 
     @Override