Enable strictPropertyInitialization (#1909)

## Description:

Enable the tsconfig option `strictPropertyInitialization`.

Fixes #1907

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

src/core/execution/ShellExecution.ts

@@ -5,11 +5,11 @@ import { TileRef } from "../game/GameMap";
 
 export class ShellExecution implements Execution {
   private active = true;
-  private pathFinder: AirPathFinder;
+  private pathFinder: AirPathFinder | undefined;
   private shell: Unit | undefined;
-  private mg: Game;
+  private mg: Game | undefined;
   private destroyAtTick = -1;
-  private random: PseudoRandom;
+  private random: PseudoRandom | undefined;
 
   constructor(
     private readonly spawn: TileRef,
@@ -25,6 +25,8 @@ export class ShellExecution implements Execution {
   }
 
   tick(ticks: number): void {
+    if (this.mg === undefined) throw new Error("Not initialized");
+    if (this.pathFinder === undefined) throw new Error("Not initialized");
     this.shell ??= this._owner.buildUnit(UnitType.Shell, this.spawn, {});
     if (!this.shell.isActive()) {
       this.active = false;
@@ -62,6 +64,8 @@ export class ShellExecution implements Execution {
   }
 
   private effectOnTarget(): number {
+    if (this.mg === undefined) throw new Error("Not initialized");
+    if (this.random === undefined) throw new Error("Not initialized");
     const { damage } = this.mg.config().unitInfo(UnitType.Shell);
     const baseDamage = damage ?? 250;