Assert _getmp() does not return None

Tests/test_file_mpo.py

@@ -156,6 +156,7 @@ def test_reload_exif_after_seek() -> None:
 def test_mp(test_file: str) -> None:
     with Image.open(test_file) as im:
         mpinfo = im._getmp()
+        assert mpinfo is not None
         assert mpinfo[45056] == b"0100"
         assert mpinfo[45057] == 2
 
@@ -165,6 +166,7 @@ def test_mp_offset() -> None:
     # in APP2 data, in contrast to normal 8
     with Image.open("Tests/images/sugarshack_ifd_offset.mpo") as im:
         mpinfo = im._getmp()
+        assert mpinfo is not None
         assert mpinfo[45056] == b"0100"
         assert mpinfo[45057] == 2
 
@@ -181,6 +183,7 @@ def test_mp_no_data() -> None:
 def test_mp_attribute(test_file: str) -> None:
     with Image.open(test_file) as im:
         mpinfo = im._getmp()
+    assert mpinfo is not None
     for frame_number, mpentry in enumerate(mpinfo[0xB002]):
         mpattr = mpentry["Attribute"]
         if frame_number: