Prevent crash when missing datum

(This can happen if the user mouseups on a tooltip, e.g. in walkthrough)

modules/behavior/select.js

@@ -74,8 +74,8 @@ export function behaviorSelect(context) {
             mode = context.mode();
 
 
-        if (datum.type === 'midpoint') {
-            // clicked midpoint, do nothing..
+        if (!datum || datum.type === 'midpoint') {
+            // do nothing..
 
         } else if (!(datum instanceof osmEntity)) {
             // clicked nothing..