fix(prefs): Fix selector for default account for send

internal_packages/preferences/lib/tabs/sending-section.cjsx

@@ -15,14 +15,14 @@ class SendingSection extends React.Component
     values = accounts.map (acc) -> acc.id
     labels = accounts.map (acc) -> acc.me().toString()
 
-    values = [null, values...]
+    values = ['selected-mailbox', values...]
     labels = ['Account of selected mailbox', labels...]
 
     _.extend(configSchema.properties.sending.properties, {
       defaultAccountIdForSend:
         type: 'string'
         title: 'Send new messages from'
-        default: null
+        default: 'selected-mailbox'
         enum: values
         enumLabels: labels
     })

src/flux/stores/draft-store.coffee

@@ -371,8 +371,9 @@ class DraftStore
 
   _getAccountForNewMessage: =>
     defAccountId = NylasEnv.config.get('core.sending.defaultAccountIdForSend')
-    if defAccountId?
-      AccountStore.accountForId(defAccountId)
+    account = AccountStore.accountForId(defAccountId)
+    if account
+      account
     else
       focusedAccountId = FocusedPerspectiveStore.current().accountIds[0]
       if focusedAccountId