Here is the FAQ page of the UML Tools.
This problem is due to strange manipulations while creating your diagram. You have selected the model node intead of selecting the main object (main package, main state, main component, etc.). Unfortunately, as model node has package type, the wizard will let you create a diagram from for this node. So, the diagram editor will think that all object have been imported from other package. This will be fixed in future versions. The following image shows you the bad manipulation
What you have to do is to be sure you have selected
the main object and not the root object which is the
model node. Selectiong the main object will change
the items inside the diagram suggestion combo box.
You have to proceed as shown in the following
picture