Fix purchase order import form

InvenTree/common/files.py

@@ -99,13 +99,18 @@ class FileManager:
         self.update_headers()
 
     def guess_header(self, header, threshold=80):
-        """ Try to match a header (from the file) to a list of known headers
+        """
+        Try to match a header (from the file) to a list of known headers
 
         Args:
             header - Header name to look for
             threshold - Match threshold for fuzzy search
         """
 
+        # Replace null values with empty string
+        if header is None:
+            header = ''
+
         # Try for an exact match
         for h in self.HEADERS:
             if h == header: