Added text input prevention for number input fields in StartView

src/main/java/millions/view/StartView.java

@@ -7,6 +7,7 @@
 import javafx.scene.control.Button;
 import javafx.scene.control.Label;
 import javafx.scene.control.TextField;
+import javafx.scene.control.TextFormatter;
 import javafx.scene.layout.VBox;
 import javafx.stage.FileChooser;
 import javafx.stage.Stage;
@@ -37,11 +38,23 @@ public StartView(Stage stage) {
     startingAmountField
         .textProperty()
         .addListener((obs, oldVal, newVal) -> checkStartButtonValid());
+    startingAmountField.setTextFormatter(new TextFormatter<>(change -> {
+      if (change.getControlNewText().matches("([0-9]*)?")) {
+        return change;
+      }
+      return null;
+    }));
     // Pre run weeks to run simulated weeks before the player starts
     preRunWeeksField = new TextField("12");
     preRunWeeksField.setPromptText("Pre run weeks:");
     preRunWeeksField.setMaxWidth(250);
     preRunWeeksField.textProperty().addListener((obs, oldVal, newVal) -> checkStartButtonValid());
+    preRunWeeksField.setTextFormatter(new TextFormatter<>(change -> {
+      if (change.getControlNewText().matches("([0-9]*)?")) {
+        return change;
+      }
+      return null;
+    }));
 
     filepickerButton = new Button();
     filepickerButton.setText("Pick file");