Fix UI canvas save as start folder (#5756)

Signed-off-by: Steve Pham <82231385+spham-amzn@users.noreply.github.com>

Gems/LyShine/Code/Editor/EditorWindow.cpp

@@ -8,6 +8,7 @@
 #include "EditorCommon.h"
 #include "CanvasHelpers.h"
 #include "AssetDropHelpers.h"
+#include <AzCore/IO/Path/Path.h>
 #include <AzCore/IO/SystemFile.h>
 #include <AzCore/std/sort.h>
 #include <AzToolsFramework/API/EditorAssetSystemAPI.h>
@@ -697,15 +698,36 @@ bool EditorWindow::SaveCanvasToXml(UiCanvasMetadata& canvasMetadata, bool forceA
         else if (recentFiles.size() > 0)
         {
             dir = Path::GetPath(recentFiles.front());
-            dir.append(canvasMetadata.m_canvasDisplayName.c_str());
         }
         // Else go to the default canvas directory
         else
         {
             dir = FileHelpers::GetAbsoluteDir(UICANVASEDITOR_CANVAS_DIRECTORY);
-            dir.append(canvasMetadata.m_canvasDisplayName.c_str());
         }
 
+        // Make sure the directory exists. If not, walk up the directory path until we find one that does
+        // so that we will have a consistent 'starting folder' in the 'AzQtComponents::FileDialog::GetSaveFileName' call
+        // across different platforms. 
+        AZ::IO::FixedMaxPath dirPath(dir.toUtf8().constData());
+
+        while (!AZ::IO::SystemFile::IsDirectory(dirPath.c_str()))
+        {
+            AZ::IO::PathView parentPath = dirPath.ParentPath();
+            if (parentPath == dirPath)
+            {
+                // We've reach the root path, need to break out whether or not
+                // the root path exists
+                break;
+            }
+            else
+            {
+                dirPath = parentPath;
+            }
+        }
+        // Append the default filename
+        dirPath /= canvasMetadata.m_canvasDisplayName;
+        dir = QString::fromUtf8(dirPath.c_str(), static_cast<int>(dirPath.Native().size()));
+
         QString filename = AzQtComponents::FileDialog::GetSaveFileName(nullptr,
             QString(),
             dir,