Drawing areas fails if no priority block exists

**Describe the bug**

Drawing an area fails silently if no `priority` block is present in the custom model.

Console log

```
Uncaught TypeError: r.priority is undefined
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
finishDrawing http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleUpEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleMapBrowserEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handlePointerUp_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
zd http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handlePointerDown_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
zd http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
Iy http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleTargetChanged_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
notify http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
set http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
setTarget http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
_o http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
il http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
ku http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
wu http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
w http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
P http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
1196 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
2851 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
7799 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
3961 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
4470 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
bundle.c784ce2a89264a26b504.js:2:1348102
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
finishDrawing http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleUpEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleMapBrowserEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handlePointerUp_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
(Async: EventListener.handleEvent)
zd http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handlePointerDown_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
(Async: EventListener.handleEvent)
zd http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
Iy http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
handleTargetChanged_ http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
dispatchEvent http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
notify http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
set http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
setTarget http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
_o http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
il http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
ku http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
wu http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
w http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
P http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
(Async: EventHandlerNonNull)
1196 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
2851 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
7799 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
3961 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
4470 http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
n http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
http://localhost:8989/maps/bundle.c784ce2a89264a26b504.js:2
```

**To Reproduce**

Open the max speed example in the custom model editor and try to draw an area.

**Expected behavior**

Drawing an area should modify an existing priority block or create a new one if it doesn't exist yet.