Hello ,
The big problem here is that you have two applications running in separate threads. Since Windows runs each thread in its own protected memory area, you can not directly access the other application's objects. You can access them if you create or start the application from the MS Project code.
This macro will check all the top level Windows (applications) that are running. It will match the caption of the window you are looking for and return true if found or false if not found. Copy this code into a separate VBA module in your project. You can then call it from anywhere else in your code.
A word of caution: If you have more than one workbook open in a single instance of Excel, the macro will fail if the workbook is not the active workbook. If you need to know if a specific workbook if open, let me know. I have code for that also.
Example
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