better derived formula management

TODO.md

@@ -64,7 +64,7 @@
   - This button will create a DerivedFormula, with the input variable as output, and the rest of the input variables as inputs.
   - The DerivedFormula will then be displayed in the FormulaScreen
 - [R] If the Formula displayed in FormulaScreen is a DerivedFormula, the edit button will be disabled
-- [ ] When a formula is derived in FormulaScreen, the new FormulaScreen is not pushed in navigator, it replacles the current FormulaScreen
-- [ ] In FormulaScreen, a Formula cant be derived if DerivedFormula.isDerivable() returns false
+- [R] When a formula is derived in FormulaScreen, the new FormulaScreen is not pushed in navigator, it replacles the current FormulaScreen
+- [R] In FormulaScreen, a Formula cant be derived if DerivedFormula.isDerivable() returns false
 - [ ] The algorithm of formulaSolver should be https://en.wikipedia.org/wiki/Newton%27s_method
 - [ ] Add a uuid column to the table or FormulaElements, so it is not necessary to load all the formulas to find a formula by uuid. This will improve performance when updating and deleting.

lib/ai/formula_screen.dart

@@ -468,6 +468,18 @@ class _FormulaScreenState extends State<FormulaScreen> {
       return;
     }
 
+    // Check if the formula can be derived
+    if (!DerivedFormula.isDerivable(formula)) {
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(
+          content: Text('This formula cannot be derived because it contains non number variables'),
+          duration: Duration(seconds: 2),
+        ),
+      );
+      return;
+    }
+
+
     try {
       // Create a DerivedFormula with this input variable as output
       final derivedFormula = DerivedFormula(
@@ -475,8 +487,8 @@ class _FormulaScreenState extends State<FormulaScreen> {
         originalFormula: formula,
       );
 
-      // Navigate to the new DerivedFormula screen
-      Navigator.push(
+      // Replace the current FormulaScreen with the new DerivedFormula screen
+      Navigator.pushReplacement(
         context,
         MaterialPageRoute(
           builder: (context) => FormulaScreen(