fix: Add stage initialization check in _onResize

[BeNikk]

Description

fixes #4300
This PR adds a defensive check in the _onResize method to ensure this.stage is properly initialized before executing resize operations. This prevents potential errors during the application's initialization phase.

Changes Made

• Added initialization check at the start of _onResize method
• Added early return if stage is not initialized
• Maintains existing resize functionality when stage is ready

Before

The _onResize method would attempt to access this.stage properties before initialization was complete.

After

Method is only called when the stage is available

Testing Done

• Verified application loads without errors
• Confirmed resize functionality works as expected after initialization
• Tested resize behavior across different browsers

[BeNikk]

@walterbender Could you please take a look, and let me know if any changes are needed