Avoid crashes when opening WebView from reader

Also ensure WebViewActivity has an Assistant URL when it first opens with a URL.

app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderActivity.kt

@@ -67,6 +67,7 @@ import eu.kanade.tachiyomi.ui.reader.viewer.ReaderProgressIndicator
 import eu.kanade.tachiyomi.ui.reader.viewer.pager.R2LPagerViewer
 import eu.kanade.tachiyomi.ui.webview.WebViewActivity
 import eu.kanade.tachiyomi.util.Constants
+import eu.kanade.tachiyomi.util.lang.launchIO
 import eu.kanade.tachiyomi.util.lang.launchNonCancellable
 import eu.kanade.tachiyomi.util.lang.withUIContext
 import eu.kanade.tachiyomi.util.preference.toggle
@@ -716,10 +717,12 @@ class ReaderActivity : BaseActivity() {
     private fun openChapterInWebview() {
         val manga = viewModel.manga ?: return
         val source = viewModel.getSource() ?: return
-        val url = viewModel.getChapterUrl() ?: return
-
-        val intent = WebViewActivity.newIntent(this, url, source.id, manga.title)
-        startActivity(intent)
+        lifecycleScope.launchIO {
+            viewModel.getChapterUrl()?.let { url ->
+                val intent = WebViewActivity.newIntent(this@ReaderActivity, url, source.id, manga.title)
+                withUIContext { startActivity(intent) }
+            }
+        }
     }
 
     private fun showReadingModeToast(mode: Int) {

app/src/main/java/eu/kanade/tachiyomi/ui/webview/WebViewActivity.kt

@@ -42,10 +42,11 @@ class WebViewActivity : BaseActivity() {
             return
         }
 
-        val url = intent.extras!!.getString(URL_KEY) ?: return
+        val url = intent.extras?.getString(URL_KEY) ?: return
+        assistUrl = url
+
         var headers = mutableMapOf<String, String>()
-        val source = sourceManager.get(intent.extras!!.getLong(SOURCE_KEY)) as? HttpSource
-        if (source != null) {
+        (sourceManager.get(intent.extras!!.getLong(SOURCE_KEY)) as? HttpSource)?.let { source ->
             headers = source.headers.toMultimap().mapValues { it.value.getOrNull(0) ?: "" }.toMutableMap()
         }