check for null mainFrameUrl

auditors: @bbondy @diracdeltas

fix #4325

Test Plan:
Test the crash
1. Open the browser with at least two tabs
2. Go to https://jpn.privateinternetaccess.com/
3. You will see the page header appear and as soon as you see the rest of the page start to load immediately close the tab
Because the timing is tricky this should be tried several times

Test ad block and tracking protection still work
1. Go to slashdot.org
2. No ads should be displayed
3. Open the dev tools and check to see that ga.js is blocked (google analytics)

Test site hacks still work
1. Go to forbes.com
2. You should go directly to the home page and not see the landing page

Test https everywhere still works
1. Go to https://badssl.com/
2. Click on https-everwhere
3. You should get a green page with ssl enabled

app/adBlock.js

@@ -28,7 +28,14 @@ const whitelistHosts = ['disqus.com', 'a.disquscdn.com']
 
 const startAdBlocking = (adblock, resourceName, shouldCheckMainFrame) => {
   Filtering.registerBeforeRequestFilteringCB((details) => {
-    const firstPartyUrl = URL.parse(Filtering.getMainFrameUrl(details))
+    const mainFrameUrl = Filtering.getMainFrameUrl(details)
+    // this can happen if the tab is closed and the webContents is no longer available
+    if (!mainFrameUrl) {
+      return {
+        resourceName: module.exports.resourceName
+      }
+    }
+    const firstPartyUrl = URL.parse(mainFrameUrl)
     let firstPartyUrlHost = firstPartyUrl.hostname || ''
     const urlHost = URL.parse(details.url).hostname
     const cancel = firstPartyUrl.protocol &&

app/httpsEverywhere.js

@@ -114,8 +114,9 @@ function startHttpsEverywhere () {
 
 function onBeforeHTTPRequest (details) {
   let result = { resourceName: module.exports.resourceName }
-  if (!Filtering.isResourceEnabled(module.exports.resourceName,
-    Filtering.getMainFrameUrl(details))) {
+
+  const mainFrameUrl = Filtering.getMainFrameUrl(details)
+  if (!mainFrameUrl || !Filtering.isResourceEnabled(module.exports.resourceName, mainFrameUrl)) {
     return result
   }
   // Ignore URLs that are not HTTP
@@ -137,8 +138,8 @@ function onBeforeHTTPRequest (details) {
 }
 
 function onBeforeRedirect (details) {
-  if (!Filtering.isResourceEnabled(module.exports.resourceName,
-    Filtering.getMainFrameUrl(details))) {
+  const mainFrameUrl = Filtering.getMainFrameUrl(details)
+  if (!mainFrameUrl || !Filtering.isResourceEnabled(module.exports.resourceName, mainFrameUrl)) {
     return
   }
 

app/siteHacks.js

@@ -26,7 +26,14 @@ module.exports.init = () => {
     let hack = siteHacks[domain]
     let customCookie
     let cancel
-    const firstPartyUrl = Filtering.getMainFrameUrl(details)
+    const mainFrameUrl = Filtering.getMainFrameUrl(details)
+    // this can happen if the tab is closed and the webContents is no longer available
+    if (!mainFrameUrl) {
+      return {
+        resourceName: module.exports.resourceName
+      }
+    }
+    const firstPartyUrl = URL.parse(mainFrameUrl)
     if (hack && hack.onBeforeSendHeaders) {
       const result = hack.onBeforeSendHeaders.call(this, details)
       if (result && result.customCookie) {
@@ -49,7 +56,14 @@ module.exports.init = () => {
 
     let redirectURL
     let cancel
-    const firstPartyUrl = Filtering.getMainFrameUrl(details)
+    const mainFrameUrl = Filtering.getMainFrameUrl(details)
+    // this can happen if the tab is closed and the webContents is no longer available
+    if (!mainFrameUrl) {
+      return {
+        resourceName: module.exports.resourceName
+      }
+    }
+    const firstPartyUrl = URL.parse(mainFrameUrl)
     if (hack && hack.onBeforeRequest &&
         (hack.enableForAll ||
          hack.enableForAdblock && Filtering.isResourceEnabled(appConfig.resourceNames.ADBLOCK, firstPartyUrl) ||

app/trackingProtection.js

@@ -21,7 +21,14 @@ const whitelistHosts = ['connect.facebook.net', 'connect.facebook.com', 'staticx
 
 const startTrackingProtection = (wnd) => {
   Filtering.registerBeforeRequestFilteringCB((details) => {
-    const firstPartyUrl = URL.parse(Filtering.getMainFrameUrl(details))
+    const mainFrameUrl = Filtering.getMainFrameUrl(details)
+    // this can happen if the tab is closed and the webContents is no longer available
+    if (!mainFrameUrl) {
+      return {
+        resourceName: module.exports.resourceName
+      }
+    }
+    const firstPartyUrl = URL.parse(mainFrameUrl)
     let firstPartyUrlHost = firstPartyUrl.hostname || ''
     if (firstPartyUrlHost.startsWith('www.')) {
       firstPartyUrlHost = firstPartyUrlHost.substring(4)