When reviewing an email don't pass characters on

This stops characters being passed to the focused box when reviewing an
email. To edit headers the user should go back to the edit page.

widgets/compose.go

@@ -177,7 +177,10 @@ func (c *Composer) Bindings() string {
 }
 
 func (c *Composer) Event(event tcell.Event) bool {
-	return c.focusable[c.focused].Event(event)
+	if c.editor != nil {
+		return c.focusable[c.focused].Event(event)
+	}
+	return false
 }
 
 func (c *Composer) Focus(focus bool) {