ROB: Raise PdfReadError when missing /Root in trailer (#2808)

Fixes #2806.

pypdf/_reader.py

@@ -190,7 +190,10 @@ def close(self) -> None:
     @property
     def root_object(self) -> DictionaryObject:
         """Provide access to "/Root". Standardized with PdfWriter."""
-        return cast(DictionaryObject, self.trailer[TK.ROOT].get_object())
+        root = self.trailer[TK.ROOT]
+        if root is None:
+            raise PdfReadError('Cannot find "/Root" key in trailer')
+        return cast(DictionaryObject, root.get_object())
 
     @property
     def _info(self) -> Optional[DictionaryObject]:

tests/test_reader.py

@@ -607,9 +607,9 @@ def test_read_unknown_zero_pages(caplog):
         "startxref on same line as offset",
     ]
     assert normalize_warnings(caplog.text) == warnings
-    with pytest.raises(AttributeError) as exc:
+    with pytest.raises(PdfReadError) as exc:
         len(reader.pages)
-    assert exc.value.args[0] == "'NoneType' object has no attribute 'get_object'"
+    assert exc.value.args[0] == 'Cannot find "/Root" key in trailer'
 
 
 def test_read_encrypted_without_decryption():