ComputationGraph.feedForward Null Pointer exception.

[RobAltena]

Issue Description

Please see: deeplearning4j/deeplearning4j-examples#659

We got a developer unfriendly Null Pointer Exception after the 1.0.0-alpha release. The example has been fixed. It is not clear why the original code fails, could be a bug but the error message can be better.

[AlexDBlack]

It's definitely not a bug; there was some changes to how input fields are cleared in some of the feed-forward methods (to avoid having invalid arrays (out of workspace scope) sitting around, which can crash the JVM if used).
I totally agree that we could have a better exception here - I'll add a check + error message in the backprop methods.

[AlexDBlack]

I've added additional checks + a useful exception here: #4900
That will be merged soon.

[lock]

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