Azure Portal AzFn Proxies Advanced editor, broken JS error

#### Investigative information

- Timestamp: 2021-06-21T09:26:59.0066024Z
- Function App version: 2.7.2936
- Function App name:
- Invocation ID: na - Azure Portal Issue not a Fn issue
- Region: Australia Southeast

#### Repro steps

Edit Proxies
Edit Proxy
Click Advance editor
JS error in Chrome and Edge

#### Expected behavior

[Expected behaviour would be as per docs](https://docs.microsoft.com/en-us/azure/azure-functions/functions-proxies#advanced-configuration)

#### Actual behavior

[WebsitesExtension] 5:20:49 pm MsPortalFx.Base.Diagnostics.ErrorReporter 3 MsPortalFx.Base.Diagnostics.ErrorReporter: message: Unexpected token o in JSON at position 1
stack: SyntaxError: Unexpected token o in JSON at position 1
at JSON.parse ()
at u.t._switchContextMenu (https://afd.hosting.portal.azure.net/websites/Content/Dynamic/OP-5r_fljZd5.js:3:6831)
at https://portal.azure.com/Content/Dynamic/wOh11kcBVcvk.js:91:1851
at Array.forEach ()
at n.callback (https://portal.azure.com/Content/Dynamic/wOh11kcBVcvk.js:91:1830)
at Function.notifySubscribers (https://portal.azure.com/Content/Dynamic/wOh11kcBVcvk.js:52:1589)
at Function.di.notifySubscribers (https://portal.azure.com/Content/Dynamic/uyYvEeA5RKSJ.js:35:17633)
at Function.valueHasMutated (https://portal.azure.com/Content/Dynamic/wOh11kcBVcvk.js:52:3341)
at i (https://portal.azure.com/Content/Dynamic/wOh11kcBVcvk.js:52:3063)
at iu (https://portal.azure.com/Content/Dynamic/uyYvEeA5RKSJ.js:35:3929)
From RPC: fx -> WebsitesExtension

#### Known workarounds

Will try again tomorrow, if it is a regional cache issue...

No repository file or test is identified. Reproduce the failure by editing an Azure Function Proxy and opening the Advanced editor in Chrome or Edge, then compare the behavior with the linked documentation; done means the editor opens without the reported JSON.parse error.

Written by the indexing model from the issue text.