FormBuilder - PHP notice Trying to get property of non-object
I had the same issue. My problem stemmed from the fact that I had put all of the form fields that I created into the field layout for my form page.



I was used to putting all the fields directly on to the page. Once I removed all the form fields from my field layout, I was able to successfully access the page to manage my content. 

It seems as though your fields for forms should be treated separately and not put directly into the page. They should be added through the formbuilder plugin.