fix: no more error popup when cancelling image import

🚀 Pull Request Summary 🚀

This pull request brings some really smart improvements to the handling of images and the structure of the code in mainwindow.cpp. Here's how: 😃

Enhanced Validation For Image Files 📸 🕵️ Now, our software first checks if a filename is not blank before creating an QImageReader. This means it won't try to read an image if a filename isn't provided, reducing unnecessary work!
Intelligent Check Before Image Processing 💾🔍 We've added a condition to verify if the QImageReader can actually read the image file before starting to work on it. This helps us avoid running into trouble midway through the process.
Helpful Error Handling ⚠️ 🎗️ If our QImageReader bumps into an image file, it can't handle, it's not going to keep it a secret. Now, it signals a warning through debug logs and pops up a message box with a clear error message to the user. Communication is important, right? 😉
Polished Code Structure 🏗️✨ Last but in no way least, we've rearranged some parts of the code, so it's much easier to follow. This helps everyone understand what's going on more quickly, whether you're a seasoned pro or a newbie in the codebase! 👍

Huge kudos to the team for these smart updates! Keep up the great work! 👏💪🎉