Fix resetting of execution context

For #616

Author: pmeenan

internal/devtools.py

@@ -1239,7 +1239,7 @@ def execute_js(self, script, use_execution_context=False):
 
     def set_execution_context(self, target):
         """ Set the js execution context by matching id, origin or name """
-        if len(target):
+        if target is not None and len(target):
             parts = target.split('=', 1)
             if len(parts) == 2:
                 key = parts[0].strip()