Remove exception on invalid input

package.json

@@ -1,6 +1,6 @@
 {
     "name": "doomsday",
-    "version": "1.5.2",
+    "version": "1.5.3",
     "description": "Test your mastery of Conway's Doomsday rule.",
     "author": "Florine W. Dekker",
     "browser": "dist/bundle.js",
@@ -29,7 +29,7 @@
         "grunt-text-replace": "^0.4.0",
         "grunt-webpack": "^5.0.0",
         "ts-loader": "^9.4.2",
-        "typescript": "^4.9.3",
+        "typescript": "^4.9.4",
         "webpack": "^5.75.0",
         "webpack-cli": "^5.0.1"
     }

src/main/js/Main.ts

@@ -75,31 +75,31 @@ class ValidatableInput {
     /**
      * Returns `true` if and only if the input is valid.
      *
-     * This method **must** be implemented by subclasses.
+     * Returns `false` by default. Implement this method to make it do something else.
      *
      * @param value the value of the input to validate
      * @return `true` if and only if the input is valid
      */
     isValid(value: string): boolean {
-        throw new Error("Implement this method.");
+        return false;
     }
 
     /**
      * Runs when the user submits a valid input.
      *
-     * This method **must** be implemented by subclasses.
+     * Does nothing by default. Implement this method to make it do something.
      */
     onValidInput(): void {
-        throw new Error("Implement this method.");
+        // Do nothing
     }
 
     /**
      * Runs when a user submits an invalid input.
      *
-     * This method **must** be implemented by subclasses.
+     * Does nothing by default. Implement this method to make it do something.
      */
     onInvalidInput(): void {
-        throw new Error("Implement this method.");
+        // Do nothing
     }