Fix regression in new confirmation Dialog

Summary:
Fix a JavaScript regression encapsulating the problematic part into an `if`.

Other minor changes:

- dedicate a variable for the confirmation messages to improve i18n in the future (but also to avoid 80 characters and make lint happy)
- replace `confirm` with `window.confirm` (to make lint happy)

Ref T15034
Ref D25015

Test Plan:
- surf on your local Phorge
- no JavaScript errors in console

Reviewers: bekay, Ekubischta, O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15034

Differential Revision: https://we.phorge.it/D25076

webroot/rsrc/externals/javelin/lib/Workflow.js

@@ -404,14 +404,18 @@ JX.install('Workflow', {
         }
       }
 
-      var form = JX.DOM.scry(this._root, 'form', 'jx-dialog');
-      if (form.length) {
-        JX.DOM.listen(form[0], 'keydown', null, function(e) {
-          if (e.getSpecialKey()) {
-            return;
-          }
-          JX.Stratcom.addSigil(form[0], 'dialog-keydown');
-        });
+      // Only when the response is a Dialog, check if the user
+      // is quitting with pending changes
+      if (this._root) {
+        var form = JX.DOM.scry(this._root, 'form', 'jx-dialog');
+        if (form.length) {
+          JX.DOM.listen(form[0], 'keydown', null, function(e) {
+            if (e.getSpecialKey()) {
+              return;
+            }
+            JX.Stratcom.addSigil(form[0], 'dialog-keydown');
+          });
+        }
       }
     },
     _push : function() {
@@ -546,13 +550,20 @@ JX.install('Workflow', {
         return;
       }
 
-      var form = JX.DOM.scry(active._root, 'form', 'jx-dialog');
-      if (
-        form.length &&
-        JX.Stratcom.hasSigil(form[0], 'dialog-keydown') &&
-        !confirm('Form data may have changed. Are you sure you want to close this dialog?')
-      ) {
-        return;
+      // Only when the response is a Dialog, check if the user
+      // is quitting with pending changes
+      if (active._root) {
+        var form = JX.DOM.scry(active._root, 'form', 'jx-dialog');
+        var confirmMsg =
+          'Form data may have changed. ' +
+          'Are you sure you want to close this dialog?';
+        if (
+          form.length &&
+          JX.Stratcom.hasSigil(form[0], 'dialog-keydown') &&
+          !window.confirm(confirmMsg)
+        ) {
+          return;
+        }
       }
 
       JX.Workflow._pop();