Raise ValueError when trying to save without bitmap

Tests/test_fontfile.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+import pytest
+
+from PIL import FontFile
+
+
+def test_save(tmp_path):
+    tempname = str(tmp_path / "temp.pil")
+
+    font = FontFile.FontFile()
+    with pytest.raises(ValueError):
+        font.save(tempname)

src/PIL/FontFile.py

@@ -118,6 +118,9 @@ class FontFile:
         self.compile()
 
         # font data
+        if not self.bitmap:
+            msg = "No bitmap created"
+            raise ValueError(msg)
         self.bitmap.save(os.path.splitext(filename)[0] + ".pbm", "PNG")
 
         # font metrics