owseive: Fixed key error for continuous datasets
owseive: Fixed key error for continuous datasets

When dataset with only continuous features was passes to
owsieve (after some other dataset) it crashed with key error
since `self.attrX` and `self.attrY` was not properly cleaned.
Also, since `self.warning` redraws the graph, `self.initCombos`
should be called before `self.warning`.

kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,marinkaz/orange3,qPCR4vir/orange3,marinkaz/orange3,marinkaz/orange3,marinkaz/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,kwikadi/orange3,kwikadi/orange3,kwikadi/orange3,kwikadi/orange3,marinkaz/orange3,qPCR4vir/orange3,kwikadi/orange3,cheral/orange3,cheral/orange3,qPCR4vir/orange3,marinkaz/orange3,qPCR4vir/orange3