Map distance tool can't be disabled correctly if it has only draw starting vertex on map.

[mirror-riddle]

maptalks's version and what browser you use?

maptalks: 0.43.0
Chrome: 71.0.3578.98

Issue description

If you create map distance tool and only draw staring vertex on the map, then try to disable map distance tool, the brower console will print following error:

maptalks.min.js:6 Uncaught TypeError: Cannot read property 'code' of null
    at t.e._verifyProjection (VM11840 maptalks.min.js:6)
    at t.e.getContainerExtent (VM11840 maptalks.min.js:6)
    at n.t.getContainerExtent (VM11840 maptalks.min.js:6)
    at n.t.getSize (VM11840 maptalks.min.js:6)
    at t.e._msOnDrawEnd (VM11840 maptalks.min.js:6)
    at t.i._fire (VM11840 maptalks.min.js:6)
    at t.i.fire (VM11840 maptalks.min.js:6)
    at t.i._fireEvent (VM11840 maptalks.min.js:6)
    at t._fireEvent (VM11840 maptalks.min.js:6)
    at t.endDraw (VM11840 maptalks.min.js:6)

If you have draw more than one vertex on the map before disable distance tool, it would work fine.

Please provide a reproduction URL (on any jsfiddle like site)

maptalks distance tool bug demo

[fuzhenn]

closed by #866