Create a basic calculator app that can perform addition, subtraction, multiplication, and division operations.
The app should have a user-friendly interface with buttons for digits (0-9), arithmetic operations (+, -, *, /), and an equals (=) button. Implement the logic to perform calculations based on user input. Display the input expression and result on the screen. Ensure error handling for invalid expressions (e.g., division by zero). Use Kotlin for coding and adhere to Android coding best practices.