Change get backend method for Lattice and run tests

camelot/parsers/lattice.py

@@ -140,7 +140,11 @@ class Lattice(BaseParser):
             return "convert" in methods
 
         if isinstance(backend, str):
-            if backend in BACKENDS.keys():
+            if backend not in BACKENDS.keys():
+                raise NotImplementedError(
+                    f"Unknown backend '{backend}' specified. Please use either 'poppler' or 'ghostscript'."
+                )
+
             if backend == "ghostscript":
                 warnings.warn(
                     "'ghostscript' will be replaced by 'poppler' as the default image conversion"
@@ -149,10 +153,6 @@ class Lattice(BaseParser):
                 )
 
             return BACKENDS[backend]()
-            else:
-                raise NotImplementedError(
-                    f"Unknown backend '{backend}' specified. Please use either 'poppler' or 'ghostscript'."
-                )
         else:
             if not implements_convert():
                 raise NotImplementedError(