holding that a valid guilty plea acts as a waiver to any postconviction claims of relief of any alleged errors or defects that happened before the entry of the plea, and that "[b]ecause [defendant's] claim of improprieties under the Uniform Agreement on Detainers implicates alleged errors or defects occurring prior to the entry of his plea," the claim was waived

holding that a validly entered guilty plea constituted a waiver to raise post-conviction claims of any alleged errors or defects occurring prior to the entry of the plea