I'm getting an error when I try to save a custom record, anyone get this before: "TypeError: Cannot read property \"length\" from null I've been able to save using the same field values in the UI.

sounds like you are either directly or indirectly referring to a

.length

property on a object that is not set (it is null)

Not enough info. What sort of script etc? How did you get the string you’re trying to calculate the length of?

I'm not trying to get the length of any string or object, this is happening when I'm doing a .save() of a custom record

id guess an internal error

Here is the full error, if anyone has run into this before and been able to get around it, help would be appreciated. I can save the record if it is not in dynamic mode but then some values don't get sourced. _{"type":"error.SuiteScriptError","name":"UNEXPECTED_ERROR","message":"TypeError: Cannot read property \"length\" from null (INVOCATION_WRAPPER#134)","stack":["<anonymous>(N/record/recordService.js)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.user:151)","each(N/common/pattern/iterator.js)","execute(cdp_ss_create_asn.js$2558644$debugger.user:107)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:27)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:19)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:35)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:1)"],"cause":{"type":"internal error","code":"UNEXPECTED_ERROR","details":"TypeError: Cannot read property \"length\" from null (INVOCATION_WRAPPER#134)","userEvent":null,"stackTrace":["<anonymous>(N/record/recordService.js)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.user:151)","each(N/common/pattern/iterator.js)","execute(cdp_ss_create_asn.js$2558644$debugger.user:107)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:27)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:19)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:35)","<anonymous>(cdp_ss_create_asn.js$2558644$debugger.wrap:1)"],"notifyOff":false},"id":"","notifyOff":false,"userFacing":false}_

Are there any default values that are formulas on custom record fields?

you can try the general debugging technique of removing lines of code until you find the statement(s) causing the error