Fix null pointers for non-existing types (#90)

* fix null pointers for non existing types

* Update VisitorWriter.java

Author: SentryMan

prism-core/src/main/java/io/avaje/prism/internal/VisitorWriter.java

@@ -289,7 +289,8 @@ public static void write(PrintWriter out, String packageName) {
             + "  @Override\n"
             + "  public StringBuilder visitError(ErrorType t, StringBuilder p) {\n"
             + "    kind = t.getKind();\n"
-            + "    return p;\n"
+            + "    mainType = t.toString();\n"
+            + "    return p.append(mainType);\n"
             + "  }\n"
             + "\n"
             + "  @Override\n"
@@ -413,7 +414,7 @@ public static void write(PrintWriter out, String packageName) {
             + "    if (obj == null || getClass() != obj.getClass()) return false;\n"
             + "    TypeMirrorVisitor other = (TypeMirrorVisitor) obj;\n"
             + "    return Objects.equals(fullType, other.fullType);\n"
-            + "  }"
+            + "  }\n"
             + "}");
   }
 }