fix 'state' report field (backport from bluebream version)

expert/browser/results.pt

@@ -70,7 +70,8 @@
 
 <metal:state define-macro="state">
   <tal:column define=" value python:col.getDisplayValue(row)">
-    <img tal:attributes="src string:$resourceBase${value/icon};
+    <img tal:condition="value"
+         tal:attributes="src string:$resourceBase${value/icon};
                          alt value/title;
                          title value/title" />
   </tal:column>

expert/field.py

@@ -109,6 +109,8 @@ class StateField(Field):
     def getDisplayValue(self, row):
         if IStateful.providedBy(row.context):
             stf = row.context
+        elif row.context is None:
+            return None
         else:
             stf = component.getAdapter(row.context, IStateful, 
                                         name=self.statesDefinition)