fix issue with empty headers list in envelope

buffer: before, once you removed the last entry in an envelopes
headers list the buffer tried to display an empty listbox, which fails.
This commit makes the buffer only add that list if it isn't empty

1 files changed, 4 insertions, 2 deletions

diff --git a/alot/buffers.py b/alot/buffers.py
index 93a1afb9..9bb247f2 100644
--- a/alot/buffers.py
+++ b/alot/buffers.py
@@ -116,8 +116,10 @@ class EnvelopeBuffer(Buffer):
                 for value in vlist:
                     lines.append((k, value))
 
-        self.header_wgt = widgets.HeadersList(lines)
-        displayed_widgets.append(self.header_wgt)
+        # add header list widget iff header values exists
+        if lines:
+            self.header_wgt = widgets.HeadersList(lines)
+            displayed_widgets.append(self.header_wgt)
 
         #display attachments
         lines = []