[6.5.x] Test Scenarios: NPE when running a Scenario multiple times

Open a Test Scenario (any will probably do; but those in https://github.com/nheron/testbug will definitely reproduce the issue). Click "Run Scenario", then click it again (and sometimes again). A NPE will be thrown.

The NPE is thrown here https://github.com/droolsjbpm/drools-wb/commit/b454197208aed6396e627071dae7ee3daced7a5a#diff-9fc96df71d8d356b40fc535fc5374149R284 but caused by https://github.com/droolsjbpm/drools-wb/commit/8045cc7a2b5be22c621e70c274a95abc260a50f3#diff-9fc96df71d8d356b40fc535fc5374149R254 where primitives are added as imports.

A null is returned from the DMO lookup of ModelField for the primitive.