Adds a warning when trying to save books containing the title author separator in title or author

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSave.java

@@ -94,7 +94,13 @@ public class CommandSave implements TabExecutor {
         }
 
         //Generate book filename
-        String fileName = BookHelper.getBookFile(book, player, saveToPublicFolder);
+        String fileName;
+        try {
+            fileName = BookHelper.getBookFile(book, player, saveToPublicFolder);
+        } catch (IllegalArgumentException exception) {
+            BooksWithoutBorders.sendErrorMessage(player, exception.getMessage());
+            return;
+        }
 
         //Make sure the used folders exist
         File file = new File(savePath);

src/main/java/net/knarcraft/bookswithoutborders/utility/BookHelper.java

@@ -121,13 +121,17 @@ public final class BookHelper {
      * @param book   <p>The book to get the file of</p>
      * @param player <p>The player trying to do something with the book</p>
      * @return <p>The book file</p>
+     * @throws IllegalArgumentException <p>If the book title or author contains the title author separator</p>
      */
     @NotNull
-    public static String getBookFile(@NotNull BookMeta book, @NotNull Player player, boolean isPublic) {
+    public static String getBookFile(@NotNull BookMeta book, @NotNull Player player, boolean isPublic) throws IllegalArgumentException {
         String titleAuthorSeparator = BooksWithoutBordersConfig.getTitleAuthorSeparator();
         String bookName;
         if (book.hasTitle()) {
             bookName = book.getTitle();
+            if (bookName == null) {
+                bookName = "Untitled";
+            }
         } else {
             bookName = "Untitled";
         }
@@ -140,6 +144,14 @@ public final class BookHelper {
             authorName = player.getName();
         } else {
             authorName = book.getAuthor();
+            if (authorName == null) {
+                authorName = "Unknown";
+            }
+        }
+
+        if (bookName.contains(titleAuthorSeparator) || authorName.contains(titleAuthorSeparator)) {
+            throw new IllegalArgumentException("The author or title contains the title author separator. Saving this " +
+                    "book would lead to unexpected problems.");
         }
 
         return fixName(cleanString(bookName + titleAuthorSeparator + authorName), false);