Fixes a bug where getRule gets run on a joint explanation (#300)

## What is the goal of this PR?
Explanation now works as expected when the first explanation is joint. This marks the 1.3.1 release.

## What are the changes implemented in this PR?
While introducing this fix I came to realise that the _unassigned variables_ error message will not be displayed when the target explanation is within the answers of the joint explanation. I have created an issue for it with high priority: #299

VERSION

@@ -1 +1 @@
-1.3.0
+1.3.1

package.json

@@ -165,5 +165,5 @@
  "prebuild": "node version.js",
  "unit": "node ./node_modules/jest/bin/jest.js ./test/unit"
  },
- "version": "1.3.0"
+ "version": "1.3.1"
 }

src/renderer/components/Visualiser/store/actions.js

@@ -225,9 +225,11 @@ export default {
 
  const explanation = await node.explanation();
 
- const when = await (await explanation.getRule()).getWhen();
- const relRegex = /(\$[^\s]*|;|{)(\s*?\(.*?\))/g;
+
  let isRelUnassigned = false;
+ const rule = await explanation.getRule();
+ const when = rule && await rule.getWhen();
+ const relRegex = /(\$[^\s]*|;|{)(\s*?\(.*?\))/g;
  let relMatches = relRegex.exec(when);
  while (relMatches) {
  if (!relMatches[1].includes('$')) {