Missing null check for this.boundaryElement in 'reachedUpperEdge'

[Eric Phillips]

Need to check that this.boundaryElement exists before attempting to access property this.boundaryElement.offsetTop to prevent Cannot read property 'offsetTop' of undefined error.

angular-sticky-things/projects/angular-sticky-things/src/lib/sticky-thing.directive.ts

Line 225 in f3bfae6

 const reachedUpperEdge = window.pageYOffset < (this.boundaryElement.offsetTop - marginTop); 

[Eric Phillips]

Closing as duplicate of issue #53 .