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Following a series of refactorings, MLIR Python bindings would crash if a dialect object requiring a context defined using mlir_attribute/type_subclass was constructed outside of the `ir.Context` context manager. The type caster for `MlirContext` would try using `ir.Context.current` when the default `None` value was provided to the `get`, which would also just return `None`. The caster would then attempt to obtain the MLIR capsule for that `None`, fail, but access it anyway without checking, leading to a C++ assertion failure or segfault. Guard against this case in nanobind adaptors. Also emit a warning to the user to clarify expectations, as the default message confusingly says that `None` is accepted as context and then fails with a type error. Using Python C API is currently recommended by nanobind in this case since the surrounding function must be marked `noexcept`. The corresponding test is in the PDL dialect since it is where I first observed the behavior. Core types are not using the `mlir_type_subclass` mechanism and are immune to the problem, so cannot be used for checking.