User input is not handled correctly when input is invalid in the scientific calculator

Currently, the app does not handle user input correctly when the user enters values and mathematical functions like sin, cos, tan, log, ln, x!, root, or 1/x in different orders.
For example, if the user enters a value followed by a mathematical function and hit enter then the app crashes but if user enters mathematical function first then enters the value and hit enter then the code works fine and user get correct output.
The app currently does not handle user input in a consistent or reliable way, leading to unexpected results and crashes. This issue needs to be addressed to make the app more user-friendly and reliable.