Fix for `obj.get_subplotspec()` being None

[JasonGross]

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I can't be bothered to minimize a test-case for this right now, sorry. Non-minimal test case at the bottom of https://gist.github.com/JasonGross/50bf9f6ccf8e1eff07fff2a35db703ee#file-l1-not-sparse-use-product-ipynb commit 981a0e4258b5dd566e784c00a98fbd30705f9438