Verifying that the tab exists in TabbedEditorcontainer tab map before consuming it.

Root cause:
To set the error icon code passes tab_id to the setTabIcon function on this._tabbedPane but never verified that the tab_id exists in the tab Map structure and then proceed to modify its property which causes an error.

e.g
https://imgur.com/a/4Nb0Chu
In the attached image code wants to consume tab: "tab_43" but it does not exist in the current map of tabs
then it tries to modify the properties of an undefined object triggering the error:

Uncaught (in promise) TypeError: Cannot read property '_setIcon' of undefined
    at TabbedPane.setTabIcon (TabbedPane.js:formatted)
    at Sources.TabbedEditorContainer._addLoadErrorIcon (sources_module.js:formatted)
    at eval (sources_module.js:formatted)

Fix:
Verify that the tab_id exists on the map before modifying the object.
Change-Id: I51b704597d0b1b68eee2e099d747d97339e9f59d
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1904287
Reviewed-by: Robert Paveza <Rob.Paveza@microsoft.com>
Reviewed-by: Mandy Chen <mandy.chen@microsoft.com>
Commit-Queue: Vidal Diazleal <vidorteg@microsoft.com>
diff --git a/front_end/sources/TabbedEditorContainer.js b/front_end/sources/TabbedEditorContainer.js
index 4e00fb5..2985ce7 100644
--- a/front_end/sources/TabbedEditorContainer.js
+++ b/front_end/sources/TabbedEditorContainer.js
@@ -509,7 +509,9 @@
   _addLoadErrorIcon(tabId) {
     const icon = UI.Icon.create('smallicon-error');
     icon.title = ls`Unable to load this content.`;
-    this._tabbedPane.setTabIcon(tabId, icon);
+    if (this._tabbedPane.tabView(tabId)) {
+      this._tabbedPane.setTabIcon(tabId, icon);
+    }
   }
 
   /**