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

Author: Soroush Saffari

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('$')) {