Don't crash if we get a touch event for a touch point not being tracked (#239)

Author: ryantrem

Modules/@babylonjs/react-native/ios/BabylonNativeInterop.mm

@@ -89,28 +89,34 @@ + (void)reportTouchEvent:(MTKView*)mtkView touches:(NSSet<UITouch*>*)touches eve
 
             switch (touch.phase) {
                 case UITouchPhaseBegan: {
+                    // The activeTouches array only grows, it does not shrink (to keep indices constant since they are used as pointer ids),
+                    // so look for an unused (null) array element and reuse it if found. Otherwise, add a new entry to the array.
                     NSUInteger pointerId = [activeTouches indexOfObject:[NSNull null]];
-                    if (pointerId == NSNotFound) {
+                    if (pointerId != NSNotFound) {
+                        [activeTouches replaceObjectAtIndex:pointerId withObject:touch];
+                    } else {
                         pointerId = [activeTouches count];
                         [activeTouches addObject:touch];
-                    } else {
-                        [activeTouches replaceObjectAtIndex:pointerId withObject:touch];
                     }
                     Babylon::SetTouchButtonState(static_cast<uint32_t>(pointerId), true, x, y);
                     break;
                 }
 
                 case UITouchPhaseMoved: {
                     NSUInteger pointerId = [activeTouches indexOfObject:touch];
-                    Babylon::SetTouchPosition(static_cast<uint32_t>(pointerId), x, y);
+                    if (pointerId != NSNotFound) {
+                        Babylon::SetTouchPosition(static_cast<uint32_t>(pointerId), x, y);
+                    }
                     break;
                 }
 
                 case UITouchPhaseEnded:
                 case UITouchPhaseCancelled: {
                     NSUInteger pointerId = [activeTouches indexOfObject:touch];
-                    [activeTouches replaceObjectAtIndex:pointerId withObject:[NSNull null]];
-                    Babylon::SetTouchButtonState(static_cast<uint32_t>(pointerId), false, x, y);
+                    if (pointerId != NSNotFound) {
+                        [activeTouches replaceObjectAtIndex:pointerId withObject:[NSNull null]];
+                        Babylon::SetTouchButtonState(static_cast<uint32_t>(pointerId), false, x, y);
+                    }
                     break;
                 }