Check nullity of webcontents and empty url from getURL()

#12508 is caused by empty
url which will leads to sending {cancel: true} to callback of
`session.webRequest.onBeforeRequest` and download process will be
aborted.
Also verified this won't break the issues which
#12256 fixes

fix #12508

Auditors: @bridiver, @diracdeltas, @bsclifton

app/filtering.js

@@ -863,11 +863,15 @@ module.exports.getMainFrameUrl = (details) => {
   if (details.resourceType === 'mainFrame') {
     return details.url
   }
+  let url = null
   const tab = webContents.fromTabID(details.tabId)
-  try {
-    return tab.getURL()
-  } catch (ex) {}
-  return details.firstPartyUrl || null
+  if (tab && !tab.isDestroyed()) {
+    url = tab.getURL()
+  }
+  if (!url && details.firstPartyUrl) {
+    url = details.firstPartyUrl
+  }
+  return url
 }
 
 module.exports.alwaysAllowFullscreen = () => {